Method and device for coding transform coefficient

ABSTRACT

An image decoding method according to the present document comprises the steps of: receiving a bitstream including residual information; deriving a quantized transform coefficient for a current block on the basis of the residual information included in the bitstream; deriving a residual sample for the current block on the basis of the quantized transform coefficient; and generating a reconstructed picture on the basis of the residual sample for the current block, wherein the residual information may be derived via different syntax elements depending on whether a transform has been applied to the current block.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.17/281,893, filed Mar. 31, 2021, which is a National Stage filing under35 U.S.C. 371 of International Application No. PCT/KR2019/013043, filedon Oct. 4, 2019, which claims the benefit of U.S. ProvisionalApplication No. 62/741,638, filed on Oct. 5, 2018, the contents of whichare all hereby incorporated by reference herein in their entirety.

BACKGROUND OF THE DISCLOSURE Field of the Disclosure

The present disclosure relates generally to image coding technology and,more particularly, to a method and apparatus for coding a transformcoefficient.

Related Art

Recently, the demand for high resolution, high quality image/video suchas 4K, 8K or more Ultra High Definition (UHD) image/video is increasingin various fields. As the image/video resolution or quality becomeshigher, relatively more amount of information orbits are transmittedthan for conventional image/video data. Therefore, if image/video dataare transmitted via a medium such as an existing wired/wirelessbroadband line or stored in a legacy storage medium, costs fortransmission and storage are readily increased.

Moreover, interests and demand are growing for virtual reality (VR) andartificial reality (AR) contents, and immersive media such as hologram;and broadcasting of images/videos exhibiting image/video characteristicsdifferent from those of an actual image/video, such as gameimages/videos, are also growing.

Therefore, a highly efficient image/video compression technique isrequired to effectively compress and transmit, store, or play highresolution, high quality images/videos showing various characteristicsas described above.

SUMMARY

The present disclosure is to provide a method and apparatus forimproving image coding efficiency.

The present disclosure is also to provide a method and apparatus forimproving efficiency of residual coding.

The present disclosure is also to provide a method and apparatus forimproving the efficiency of residual coding according to whether or nottransform skip is applied.

According to an embodiment of the present disclosure, there is providedan image decoding method performed by a decoding apparatus, the methodincluding: receiving a bitstream including residual information;deriving a quantized transform coefficient for a current block based onthe residual information included in the bitstream; deriving a residualsample for the current block based on the quantized transformcoefficient; and generating a reconstructed picture based on theresidual sample for the current block, wherein the residual informationmay be derived through different syntax elements depending on whether ornot a transform is applied to the current block.

The residual information includes a first transform coefficient levelflag for whether or not a transform coefficient level for the quantizedtransform coefficient is greater than a first threshold value, and asecond transform coefficient level flag for whether or not the transformcoefficient level of the quantized transform coefficient is greater thana second threshold value, and wherein the second transform coefficientlevel flag is decoded in different ways depending on whether or not atransform is applied to the current block.

The residual information includes a context syntax element coded basedon a context, and wherein the context syntax element includes asignificant coefficient flag related to whether or not the quantizedtransform coefficient is a non-zero significant coefficient, a paritylevel flag for a parity of a transform coefficient level for thequantized transform coefficient, a first transform coefficient levelflag for whether or not the transform coefficient level is greater thana first threshold value, and a second transform coefficient level flagfor whether or not the transform coefficient level of the quantizedtransform coefficient is greater than a second threshold value.

The step of deriving the quantized transform coefficient includes:decoding the first transform coefficient level flag, and decoding theparity level flag; and deriving the quantized transform coefficientbased on a value of the decoded parity level flag and a value of thedecoded first transform coefficient level flag, and wherein the decodingof the first transform coefficient level flag is performed prior to thedecoding of the parity level flag.

According to another embodiment of the present disclosure, there isprovided an image encoding method by an encoding apparatus, the methodincluding: deriving a residual sample for a current block; deriving aquantized transform coefficient based on the residual sample for thecurrent block; and encoding residual information including informationon the quantized transform coefficient, wherein the residual informationmay be derived through different syntax elements depending on whether ornot a transform is applied to the current block.

According to still another embodiment of the present disclosure, animage decoding apparatus for performing an image decoding methodincludes: an entropy decoder which receives a bitstream includingresidual information, and derives a quantized transform coefficient fora current block based on the residual information included in thebitstream; an inverse transformer which derives a residual sample forthe current block based on the quantized transform coefficient; and anadder which generates a reconstructed picture based on the residualsample for the current block, wherein the residual information may bederived through different syntax elements depending on whether or not atransform is applied to the current block.

According to still another embodiment of the present disclosure, thereis provided an encoding apparatus for performing image encoding. Theencoding apparatus includes a subtractor which derives a residual samplefor a current block, a quantizer which derives a quantized transformcoefficient based on the residual sample for the current block; and anentropy encoder which encodes residual information including informationon the quantized transform coefficient, wherein the residual informationmay be derived through different syntax elements depending on whether ornot a transform is applied to the current block.

According to still another embodiment of the present disclosure, adigital storage medium in which image data including encoded imageinformation generated according to the image encoding method performedby an encoding apparatus is stored may be provided.

According to still another embodiment of the present disclosure, adigital storage medium in which image data including encoded imageinformation causing the decoding apparatus to perform the image decodingmethod is stored may be provided.

According to an embodiment of the present disclosure, it is possible toimprove general image/video compression efficiency.

According to an embodiment of the present disclosure, it is possible toimprove the efficiency of residual coding.

According to the present disclosure, it is possible to improve theefficiency of transform coefficient coding.

According to the present disclosure, it is possible to improve theefficiency of residual coding according to whether or not transform skipis applied.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically represents an example of a video/image codingsystem to which the present disclosure may be applied.

FIG. 2 is a diagram schematically illustrating a configuration of avideo/image encoding apparatus to which the present disclosure may beapplied.

FIG. 3 is a diagram schematically illustrating a configuration of avideo/image decoding apparatus to which the present disclosure may beapplied.

FIG. 4 is a control flowchart illustrating a video/image encoding methodto which the present disclosure may be applied.

FIG. 5 is a control flowchart illustrating a video/image decoding methodto which the present disclosure may be applied.

FIG. 6 is a diagram illustrating a block diagram of a CABAC encodingsystem according to an embodiment.

FIG. 7 is a diagram illustrating an example of transform coefficients ina 4×4 block.

FIG. 8 is a diagram illustrating a residual signal decoder according toan example of the present disclosure.

FIG. 9 is a control flowchart illustrating a method of decoding aresidual signal according to an exemplary embodiment of the presentdisclosure.

FIG. 10 is a control flowchart illustrating a method of parsing acontext element according to an embodiment of the present disclosure.

FIG. 11 is a control flowchart illustrating a method of parsing acontext element according to another embodiment of the presentdisclosure.

FIG. 12 is a control flowchart illustrating a method of parsing acontext element according to still another embodiment of the presentdisclosure.

FIG. 13 is a control flowchart illustrating a method of parsing acontext element according to still another embodiment of the presentdisclosure.

FIG. 14 is a control flowchart illustrating a method of parsing acontext element according to still another embodiment of the presentdisclosure.

FIG. 15 is a control flowchart illustrating a method of parsing acontext element according to still another embodiment of the presentdisclosure.

FIG. 16 illustratively represents a content streaming system structurediagram to which the present disclosure may be applied.

DESCRIPTION OF EMBODIMENTS

The present disclosure may be modified in various forms, and specificembodiments thereof will be described and illustrated in the drawings.However, the embodiments are not intended for limiting the disclosure.The terms used in the following description are used to merely describespecific embodiments, but are not intended to limit the disclosure. Anexpression of a singular number includes an expression of the pluralnumber, so long as it is clearly read differently. The terms such as“include” and “have” are intended to indicate that features, numbers,steps, operations, elements, components, or combinations thereof used inthe following description exist and it should be thus understood thatthe possibility of existence or addition of one or more differentfeatures, numbers, steps, operations, elements, components, orcombinations thereof is not excluded.

Meanwhile, each component on the drawings described herein isillustrated independently for convenience of description as tocharacteristic functions different from each other, and however, it isnot meant that each component is realized by a separate hardware orsoftware. For example, any two or more of these components may becombined to form a single component, and any single component may bedivided into plural components. The embodiments in which components arecombined and/or divided will belong to the scope of the patent right ofthe present document as long as they do not depart from the essence ofthe present document.

Hereinafter, examples of the present embodiment will be described indetail with reference to the accompanying drawings. In addition, likereference numerals are used to indicate like elements throughout thedrawings, and the same descriptions on the like elements will beomitted.

FIG. 1 illustrates an example of a video/image coding system to whichthe present disclosure may be applied.

Referring to FIG. 1 , a video/image coding system may include a sourcedevice and a reception device. The source device may transmit encodedvideo/image information or data to the reception device through adigital storage medium or network in the form of a file or streaming.

The source device may include a video source, an encoding apparatus, anda transmitter. The receiving device may include a receiver, a decodingapparatus, and a renderer. The encoding apparatus may be called avideo/image encoding apparatus, and the decoding apparatus may be calleda video/image decoding apparatus. The transmitter may be included in theencoding apparatus. The receiver may be included in the decodingapparatus. The renderer may include a display, and the display may beconfigured as a separate device or an external component.

The video source may acquire video/image through a process of capturing,synthesizing, or generating the video/image. The video source mayinclude a video/image capture device and/or a video/image generatingdevice. The video/image capture device may include, for example, one ormore cameras, video/image archives including previously capturedvideo/images, and the like. The video/image generating device mayinclude, for example, computers, tablets and smartphones, and may(electronically) generate video/images. For example, a virtualvideo/image may be generated through a computer or the like. In thiscase, the video/image capturing process may be replaced by a process ofgenerating related data.

The encoding apparatus may encode input video/image. The encodingapparatus may perform a series of procedures such as prediction,transform, and quantization for compaction and coding efficiency. Theencoded data (encoded video/image information) may be output in the formof a bitstream.

The transmitter may transmit the encoded image/image information or dataoutput in the form of a bitstream to the receiver of the receivingdevice through a digital storage medium or a network in the form of afile or streaming. The digital storage medium may include variousstorage mediums such as USB, SD, CD, DVD, Blu-ray, HDD, SSD, and thelike. The transmitter may include an element for generating a media filethrough a predetermined file format and may include an element fortransmission through a broadcast/communication network. The receiver mayreceive/extract the bitstream and transmit the received bitstream to thedecoding apparatus.

The decoding apparatus may decode the video/image by performing a seriesof procedures such as dequantization, inverse transform, and predictioncorresponding to the operation of the encoding apparatus.

The renderer may render the decoded video/image. The renderedvideo/image may be displayed through the display.

This document relates to video/image coding. For example, amethod/embodiment disclosed in this document may be applied to a methoddisclosed in the versatile video coding (VVC) standard, the essentialvideo coding (EVC) standard, the AOMedia Video 1 (AV1) standard, the 2ndgeneration of audio video coding standard (AVS2) or the next generationvideo/image coding standard (e.g., H.267, H.268, or the like).

This document suggests various embodiments of video/image coding, andthe above embodiments may also be performed in combination with eachother unless otherwise specified.

In this document, a video may refer to a series of images over time. Apicture generally refers to the unit representing one image at aparticular time frame, and a slice/tile refers to the unit constitutinga part of the picture in terms of coding. A slice/tile may include oneor more coding tree units (CTUs). One picture may consist of one or moreslices/tiles. One picture may consist of one or more tile groups. Onetile group may include one or more tiles. A brick may represent arectangular region of CTU rows within a tile in a picture (a brick mayrepresent a rectangular region of CTU rows within a tile in a picture).A tile may be partitioned into a multiple bricks, each of which may beconstructed with one or more CTU rows within the tile (A tile may bepartitioned into multiple bricks, each of which consisting of one ormore CTU rows within the tile). A tile that is not partitioned intomultiple bricks may also be referred to as a brick. A brick scan mayrepresent a specific sequential ordering of CTUs partitioning a picture,wherein the CTUs may be ordered in a CTU raster scan within a brick, andbricks within a tile may be ordered consecutively in a raster scan ofthe bricks of the tile, and tiles in a picture may be orderedconsecutively in a raster scan of the tiles of the picture (A brick scanis a specific sequential ordering of CTUs partitioning a picture inwhich the CTUs are ordered consecutively in CTU raster scan in a brick,bricks within a tile are ordered consecutively in a raster scan of thebricks of the tile, and tiles in a picture are ordered consecutively ina raster scan of the tiles of the picture). A tile is a particular tilecolumn and a rectangular region of CTUs within a particular tile column(A tile is a rectangular region of CTUs within a particular tile columnand a particular tile row in a picture). The tile column is arectangular region of CTUs, which has a height equal to the height ofthe picture and a width that may be specified by syntax elements in thepicture parameter set (The tile column is a rectangular region of CTUshaving a height equal to the height of the picture and a width specifiedby syntax elements in the picture parameter set). The tile row is arectangular region of CTUs, which has a width specified by syntaxelements in the picture parameter set and a height that may be equal tothe height of the picture (The tile row is a rectangular region of CTUshaving a height specified by syntax elements in the picture parameterset and a width equal to the width of the picture). A tile scan mayrepresent a specific sequential ordering of CTUs partitioning a picture,and the CTUs may be ordered consecutively in a CTU raster scan in atile, and tiles in a picture may be ordered consecutively in a rasterscan of the tiles of the picture (A tile scan is a specific sequentialordering of CTUs partitioning a picture in which the CTUs are orderedconsecutively in CTU raster scan in a tile whereas tiles in a pictureare ordered consecutively in a raster scan of the tiles of the picture).A slice may include an integer number of bricks of a picture, and theinteger number of bricks may be included in a single NAL unit (A sliceincludes an integer number of bricks of a picture that may beexclusively contained in a single NAL unit). A slice may be constructedwith multiple complete tiles, or may be a consecutive sequence ofcomplete bricks of one tile (A slice may consists of either a number ofcomplete tiles or only a consecutive sequence of complete bricks of onetile). In this document, a tile group and a slice may be used in placeof each other. For example, in this document, a tile group/tile groupheader may be referred to as a slice/slice header.

A pixel or a pel may mean a smallest unit constituting one picture (orimage). Also, ‘sample’ may be used as a term corresponding to a pixel. Asample may generally represent a pixel or a value of a pixel, and mayrepresent only a pixel/pixel value of a luma component or only apixel/pixel value of a chroma component.

A unit may represent a basic unit of image processing. The unit mayinclude at least one of a specific region of the picture and informationrelated to the region. One unit may include one luma block and twochroma (ex. cb, cr) blocks. The unit may be used interchangeably withterms such as block or area in some cases. In a general case, an M×Nblock may include samples (or sample arrays) or a set (or array) oftransform coefficients of M columns and N rows.

In this document, the symbol “/” and “,” should be interpreted as“and/or.” For example, the expression “A/B” is interpreted as “A and/orB”, and the expression “A, B” is interpreted as “A and/or B.”Additionally, the expression “A/B/C” means “at least one of A, B, and/orC.” Further, the expression “A, B, C” also means “at least one of A, B,and/or C.” (In this document, the term “/“and”,” should be interpretedto indicate “and/or.” For instance, the expression “A/B” may mean “Aand/or B.” Further, “A, B” may mean “A and/or B.” Further, “A/B/C” maymean “at least one of A, B, and/or C.” Also, “A/B/C” may mean “at leastone of A, B, and/or C.”)

Additionally, in the present document, the term “or” should beinterpreted as “and/or.” For example, the expression “A or B” maymean 1) only “A”, 2) only “B”, and/or 3) “both A and B.” In other words,the term “or” in the present document may mean “additionally oralternatively.” (Further, in the document, the term “or” should beinterpreted to indicate “and/or.” For instance, the expression “A or B”may comprise 1) only A, 2) only B, and/or 3) both A and B. In otherwords, the term “or” in this document should be interpreted to indicate“additionally or alternatively.”)

FIG. 2 is a diagram schematically illustrating a configuration of avideo/image encoding apparatus to which the present document may beapplied. Hereinafter, what is referred to as the video encodingapparatus may include an image encoding apparatus.

Referring to FIG. 2 , the encoding apparatus 200 may include and beconfigured with an image partitioner 210, a predictor 220, a residualprocessor 230, an entropy encoder 240, an adder 250, a filter 260, and amemory 270. The predictor 220 may include an inter predictor 221 and anintra predictor 222. The residual processor 230 may include atransformer 232, a quantizer 233, a dequantizer 234, and an inversetransformer 235. The residual processor 230 may further include asubtractor 231. The adder 250 may be called a reconstructor orreconstructed block generator. The image partitioner 210, the predictor220, the residual processor 230, the entropy encoder 240, the adder 250,and the filter 260, which have been described above, may be configuredby one or more hardware components (e.g., encoder chipsets orprocessors) according to an embodiment. In addition, the memory 270 mayinclude a decoded picture buffer (DPB), and may also be configured by adigital storage medium. The hardware component may further include thememory 270 as an internal/external component.

The image partitioner 210 may split an input image (or, picture, frame)input to the encoding apparatus 200 into one or more processing units.As an example, the processing unit may be called a coding unit (CU). Inthis case, the coding unit may be recursively split according to aQuad-tree binary-tree ternary-tree (QTBTTT) structure from a coding treeunit (CTU) or the largest coding unit (LCU). For example, one codingunit may be split into a plurality of coding units of a deeper depthbased on a quad-tree structure, a binary-tree structure, and/or aternary-tree structure. In this case, for example, the quad-treestructure is first applied and the binary-tree structure and/or theternary-tree structure may be later applied. Alternatively, thebinary-tree structure may also be first applied. A coding procedureaccording to the present disclosure may be performed based on a finalcoding unit which is not split any more. In this case, based on codingefficiency according to image characteristics or the like, the maximumcoding unit may be directly used as the final coding unit, or asnecessary, the coding unit may be recursively split into coding units ofa deeper depth, such that a coding unit having an optimal size may beused as the final coding unit. Here, the coding procedure may include aprocedure such as prediction, transform, and reconstruction to bedescribed later. As another example, the processing unit may furtherinclude a prediction unit (PU) or a transform unit (TU). In this case,each of the prediction unit and the transform unit may be split orpartitioned from the aforementioned final coding unit. The predictionunit may be a unit of sample prediction, and the transform unit may be aunit for inducing a transform coefficient and/or a unit for inducing aresidual signal from the transform coefficient.

The unit may be interchangeably used with the term such as a block or anarea in some cases. Generally, an M×N block may represent samplescomposed of M columns and N rows or a group of transform coefficients.The sample may generally represent a pixel or a value of the pixel, andmay also represent only the pixel/pixel value of a luma component, andalso represent only the pixel/pixel value of a chroma component. Thesample may be used as the term corresponding to a pixel or a pelconfiguring one picture (or image).

The encoding apparatus 200 may generate a residual signal (residualblock, residual sample array) by subtracting a predicted signal(predicted block, prediction sample array) output from the interpredictor 221 or the intra predictor 222 from the input image signal(original block, original sample array), and the generated residualsignal is transmitted to the transformer 232. In this case, asillustrated, the unit for subtracting the predicted signal (predictedblock, prediction sample array) from the input image signal (originalblock, original sample array) within an encoder 200 may be called thesubtractor 231. The predictor may perform prediction for a block to beprocessed (hereinafter, referred to as a current block), and generate apredicted block including prediction samples of the current block. Thepredictor may determine whether intra prediction is applied or interprediction is applied in units of the current block or the CU. Thepredictor may generate various information about prediction, such asprediction mode information, to transfer the generated information tothe entropy encoder 240 as described later in the description of eachprediction mode. The information about prediction may be encoded by theentropy encoder 240 to be output in a form of the bitstream.

The intra predictor 222 may predict a current block with reference tosamples within a current picture. The referenced samples may be locatedneighboring to the current block, or may also be located away from thecurrent block according to the prediction mode. The prediction modes inthe intra prediction may include a plurality of non-directional modesand a plurality of directional modes. The non-directional mode mayinclude, for example, a DC mode or a planar mode. The directional modemay include, for example, 33 directional prediction modes or 65directional prediction modes according to the fine degree of theprediction direction. However, this is illustrative and the directionalprediction modes which are more or less than the above number may beused according to the setting. The intra predictor 222 may alsodetermine the prediction mode applied to the current block using theprediction mode applied to the neighboring block.

The inter predictor 221 may induce a predicted block of the currentblock based on a reference block (reference sample array) specified by amotion vector on a reference picture. At this time, in order to decreasethe amount of motion information transmitted in the inter predictionmode, the motion information may be predicted in units of a block, asub-block, or a sample based on the correlation of the motioninformation between the neighboring block and the current block. Themotion information may include a motion vector and a reference pictureindex. The motion information may further include inter predictiondirection (L0 prediction, L1 prediction, Bi prediction, or the like)information. In the case of the inter prediction, the neighboring blockmay include a spatial neighboring block existing within the currentpicture and a temporal neighboring block existing in the referencepicture. The reference picture including the reference block and thereference picture including the temporal neighboring block may also bethe same as each other, and may also be different from each other. Thetemporal neighboring block may be called the name such as a collocatedreference block, a collocated CU (colCU), or the like, and the referencepicture including the temporal neighboring block may also be called acollocated picture (colPic). For example, the inter predictor 221 mayconfigure a motion information candidate list based on the neighboringblocks, and generate information indicating what candidate is used toderive the motion vector and/or the reference picture index of thecurrent block. The inter prediction may be performed based on variousprediction modes, and for example, in the case of a skip mode and amerge mode, the inter predictor 221 may use the motion information ofthe neighboring block as the motion information of the current block. Inthe case of the skip mode, the residual signal may not be transmittedunlike the merge mode. A motion vector prediction (MVP) mode mayindicate the motion vector of the current block by using the motionvector of the neighboring block as a motion vector predictor, andsignaling a motion vector difference.

The predictor 200 may generate a predicted signal based on variousprediction methods to be described later. For example, the predictor maynot only apply the intra prediction or the inter prediction forpredicting one block, but also simultaneously apply the intra predictionand the inter prediction. This may be called a combined inter and intraprediction (CIIP). Further, the predictor may be based on an intra blockcopy (IBC) prediction mode, or a palette mode in order to performprediction on a block. The IBC prediction mode or palette mode may beused for content image/video coding of a game or the like, such asscreen content coding (SCC). The IBC basically performs prediction in acurrent picture, but it may be performed similarly to inter predictionin that it derives a reference block in a current picture. That is, theIBC may use at least one of inter prediction techniques described in thepresent document. The palette mode may be regarded as an example ofintra coding or intra prediction. When the palette mode is applied, asample value in a picture may be signaled based on information on apalette index and a palette table.

The predicted signal generated through the predictor (including theinter predictor 221 and/or the intra predictor 222) may be used togenerate a reconstructed signal or used to generate a residual signal.The transformer 232 may generate transform coefficients by applying thetransform technique to the residual signal. For example, the transformtechnique may include at least one of a discrete cosine transform (DCT),a discrete sine transform (DST), a Karhunen-Loeve transform (KLT), agraph-based transform (GBT), or a conditionally non-linear transform(CNT). Here, when the relationship information between pixels isillustrated as a graph, the GBT means the transform obtained from thegraph. The CNT means the transform which is acquired based on apredicted signal generated by using all previously reconstructed pixels.In addition, the transform process may also be applied to a pixel blockhaving the same size of the square, and may also be applied to the blockhaving a variable size rather than the square.

The quantizer 233 may quantize the transform coefficients to transmitthe quantized transform coefficients to the entropy encoder 240, and theentropy encoder 240 may encode the quantized signal (information aboutthe quantized transform coefficients) to the encoded quantized signal tothe bitstream. The information about the quantized transformcoefficients may be called residual information. The quantizer 233 mayrearrange the quantized transform coefficients having a block form in aone-dimensional vector form based on a coefficient scan order, and alsogenerate the information about the quantized transform coefficientsbased on the quantized transform coefficients of the one dimensionalvector form. The entropy encoder 240 may perform various encodingmethods, for example, such as an exponential Golomb coding, acontext-adaptive variable length coding (CAVLC), and a context-adaptivebinary arithmetic coding (CABAC). The entropy encoder 240 may alsoencode information (e.g., values of syntax elements and the like)necessary for reconstructing video/image other than the quantizedtransform coefficients together or separately. The encoded information(e.g., encoded video/image information) may be transmitted or stored inunits of network abstraction layer (NAL) unit in a form of thebitstream. The video/image information may further include informationabout various parameter sets such as an adaptation parameter set (APS),a picture parameter set (PPS), a sequence parameter set (SPS), or avideo parameter set (VPS). In addition, the video/image information mayfurther include general constraint information. The signaled/transmittedinformation and/or syntax elements to be described later in thisdocument may be encoded through the aforementioned encoding procedureand thus included in the bitstream. The bitstream may be transmittedthrough a network, or stored in a digital storage medium. Here, thenetwork may include a broadcasting network and/or a communicationnetwork, or the like, and the digital storage medium may include variousstorage media such as USB, SD, CD, DVD, Blue-ray, HDD, and SSD. Atransmitter (not illustrated) for transmitting the signal output fromthe entropy encoder 240 and/or a storage (not illustrated) for storingthe signal may be configured as the internal/external elements of theencoding apparatus 200, or the transmitter may also be included in theentropy encoder 240.

The quantized transform coefficients output from the quantizer 233 maybe used to generate a predicted signal. For example, the dequantizer 234and the inverse transformer 235 apply dequantization and inversetransform to the quantized transform coefficients, such that theresidual signal (residual block or residual samples) may bereconstructed. The adder 250 adds the reconstructed residual signal tothe predicted signal output from the inter predictor 221 or the intrapredictor 222, such that the reconstructed signal (reconstructedpicture, reconstructed block, reconstructed sample array) may begenerated. As in the case where the skip mode is applied, if there is noresidual for the block to be processed, the predicted block may be usedas the reconstructed block. The adder 250 may be called a reconstructoror a reconstructed block generator. The generated reconstructed signalmay be used for the intra prediction of the next block to be processedwithin the current picture, and as described later, also used for theinter prediction of the next picture through filtering.

Meanwhile, a luma mapping with chroma scaling (LMCS) may also be appliedin a picture encoding and/or reconstruction process.

The filter 260 may apply filtering to the reconstructed signal, therebyimproving subjective/objective image qualities. For example, the filter260 may apply various filtering methods to the reconstructed picture togenerate a modified reconstructed picture, and store the modifiedreconstructed picture in the memory 270, specifically, the DPB of thememory 270. Various filtering methods may include, for example, adeblocking filtering, a sample adaptive offset, an adaptive loop filter,a bilateral filter, and the like. The filter 260 may generate variousfiltering-related information to transfer the generated information tothe entropy encoder 240, as described later in the description of eachfiltering method. The filtering-related information may be encoded bythe entropy encoder 240 to be output in a form of the bitstream.

The modified reconstructed picture transmitted to the memory 270 may beused as the reference picture in the inter predictor 221. If the interprediction is applied by the inter predictor, the encoding apparatus mayavoid the prediction mismatch between the encoding apparatus 200 and thedecoding apparatus, and also improve coding efficiency.

The DPB of the memory 270 may store the modified reconstructed pictureto be used as the reference picture in the inter predictor 221. Thememory 270 may store motion information of the block in which the motioninformation within the current picture is derived (or encoded) and/ormotion information of the blocks within the previously reconstructedpicture. The stored motion information may be transferred to the interpredictor 221 to be utilized as motion information of the spatialneighboring block or motion information of the temporal neighboringblock. The memory 270 may store the reconstructed samples of thereconstructed blocks within the current picture, and transfer thereconstructed samples to the intra predictor 222.

FIG. 3 is a diagram for schematically explaining a configuration of avideo/image decoding apparatus to which the present disclosure isapplicable.

Referring to FIG. 3 , the decoding apparatus 300 may include andconfigured with an entropy decoder 310, a residual processor 320, apredictor 330, an adder 340, a filter 350, and a memory 360. Thepredictor 330 may include an inter predictor 331 and an intra predictor332. The residual processor 320 may include a dequantizer 321 and aninverse transformer 322. The entropy decoder 310, the residual processor320, the predictor 330, the adder 340, and the filter 350, which havebeen described above, may be configured by one or more hardwarecomponents (e.g., decoder chipsets or processors) according to anembodiment. Further, the memory 360 may include a decoded picture buffer(DPB), and may be configured by a digital storage medium. The hardwarecomponent may further include the memory 360 as an internal/externalcomponent.

When the bitstream including the video/image information is input, thedecoding apparatus 300 may reconstruct the image in response to aprocess in which the video/image information is processed in theencoding apparatus illustrated in FIG. 2 . For example, the decodingapparatus 300 may derive the units/blocks based on block split-relatedinformation acquired from the bitstream. The decoding apparatus 300 mayperform decoding using the processing unit applied to the encodingapparatus. Therefore, the processing unit for the decoding may be, forexample, a coding unit, and the coding unit may be split according tothe quad-tree structure, the binary-tree structure, and/or theternary-tree structure from the coding tree unit or the maximum codingunit. One or more transform units may be derived from the coding unit.In addition, the reconstructed image signal decoded and output throughthe decoding apparatus 300 may be reproduced through a reproducingapparatus.

The decoding apparatus 300 may receive the signal output from theencoding apparatus illustrated in FIG. 2 in a form of the bitstream, andthe received signal may be decoded through the entropy decoder 310. Forexample, the entropy decoder 310 may derive information (e.g.,video/image information) necessary for the image reconstruction (orpicture reconstruction) by parsing the bitstream. The video/imageinformation may further include information about various parameter setssuch as an adaptation parameter set (APS), a picture parameter set(PPS), a sequence parameter set (SPS), and a video parameter set (VPS).In addition, the video/image information may further include generalconstraint information. The decoding apparatus may decode the picturefurther based on the information about the parameter set and/or thegeneral constraint information. The signaled/received information and/orsyntax elements to be described later in this document may be decodedthrough the decoding procedure and acquired from the bitstream. Forexample, the entropy decoder 310 may decode information within thebitstream based on a coding method such as an exponential Golomb coding,a CAVLC, or a CABAC, and output a value of the syntax element necessaryfor the image reconstruction, and the quantized values of theresidual-related transform coefficient. More specifically, the CABACentropy decoding method may receive a bin corresponding to each syntaxelement from the bitstream, determine a context model using syntaxelement information to be decoded and decoding information of theneighboring block and the block to be decoded or information of thesymbol/bin decoded in the previous stage, and generate a symbolcorresponding to a value of each syntax element by predicting theprobability of generation of the bin according to the determined contextmodel to perform the arithmetic decoding of the bin. At this time, theCABAC entropy decoding method may determine the context model and thenupdate the context model using the information of the decoded symbol/binfor a context model of a next symbol/bin. The information aboutprediction among the information decoded by the entropy decoder 310 maybe provided to the predictor (the inter predictor 332 and the intrapredictor 331), and a residual value at which the entropy decoding isperformed by the entropy decoder 310, that is, the quantized transformcoefficients and the related parameter information may be input to theresidual processor 320. The residual processor 320 may derive a residualsignal (residual block, residual samples, residual sample array). Inaddition, the information about filtering among the information decodedby the entropy decoder 310 may be provided to the filter 350. Meanwhile,a receiver (not illustrated) for receiving the signal output from theencoding apparatus may be further configured as the internal/externalelement of the decoding apparatus 300, or the receiver may also be acomponent of the entropy decoder 310. Meanwhile, the decoding apparatusaccording to this document may be called a video/image/picture decodingapparatus, and the decoding apparatus may also be classified into aninformation decoder (video/image/picture information decoder) and asample decoder (video/image/picture sample decoder). The informationdecoder may include the entropy decoder 310, and the sample decoder mayinclude at least one of the dequantizer 321, the inverse transformer322, the adder 340, the filter 350, the memory 360, the inter predictor332, and the intra predictor 331.

The dequantizer 321 may dequantize the quantized transform coefficientsto output the transform coefficients. The dequantizer 321 may rearrangethe quantized transform coefficients in a two-dimensional block form. Inthis case, the rearrangement may be performed based on a coefficientscan order performed by the encoding apparatus. The dequantizer 321 mayperform dequantization for the quantized transform coefficients using aquantization parameter (e.g., quantization step size information), andacquire the transform coefficients.

The inverse transformer 322 inversely transforms the transformcoefficients to acquire the residual signal (residual block, residualsample array).

The predictor 330 may perform the prediction of the current block, andgenerate a predicted block including the prediction samples of thecurrent block. The predictor may determine whether the intra predictionis applied or the inter prediction is applied to the current block basedon the information about prediction output from the entropy decoder 310,and determine a specific intra/inter prediction mode.

The predictor may generate the predicted signal based on variousprediction methods to be described later. For example, the predictor maynot only apply the intra prediction or the inter prediction for theprediction of one block, but also apply the intra prediction and theinter prediction at the same time. This may be called a combined interand intra prediction (CIIP). Further, the predictor may be based on anintra block copy (IBC) prediction mode, or a palette mode in order toperform prediction on a block. The IBC prediction mode or palette modemay be used for content image/video coding of a game or the like, suchas screen content coding (SCC). The IBC basically performs prediction ina current picture, but it may be performed similarly to inter predictionin that it derives a reference block in a current picture. That is, theIBC may use at least one of inter prediction techniques described in thepresent document. The palette mode may be regarded as an example ofintra coding or intra prediction. When the palette mode is applied,information on a palette table and a palette index may be included inthe video/image information and signaled.

The intra predictor 331 may predict the current block with reference tothe samples within the current picture. The referenced samples may belocated neighboring to the current block according to the predictionmode, or may also be located away from the current block. The predictionmodes in the intra prediction may include a plurality of non-directionalmodes and a plurality of directional modes. The intra predictor 331 mayalso determine the prediction mode applied to the current block usingthe prediction mode applied to the neighboring block.

The inter predictor 332 may induce the predicted block of the currentblock based on the reference block (reference sample array) specified bythe motion vector on the reference picture. At this time, in order todecrease the amount of the motion information transmitted in the interprediction mode, the motion information may be predicted in units of ablock, a sub-block, or a sample based on the correlation of the motioninformation between the neighboring block and the current block. Themotion information may include a motion vector and a reference pictureindex. The motion information may further include inter predictiondirection (L0 prediction, L1 prediction, Bi prediction, or the like)information. In the case of the inter prediction, the neighboring blockmay include a spatial neighboring block existing within the currentpicture and a temporal neighboring block existing in the referencepicture. For example, the inter predictor 332 may configure a motioninformation candidate list based on the neighboring blocks, and derivethe motion vector and/or the reference picture index of the currentblock based on received candidate selection information. The interprediction may be performed based on various prediction modes, and theinformation about the prediction may include information indicating themode of the inter prediction of the current block.

The adder 340 may add the acquired residual signal to the predictedsignal (predicted block, prediction sample array) output from thepredictor (including the inter predictor 332 and/or the intra predictor331) to generate the reconstructed signal (reconstructed picture,reconstructed block, reconstructed sample array). As in the case wherethe skip mode is applied, if there is no residual for the block to beprocessed, the predicted block may be used as the reconstructed block.

The adder 340 may be called a reconstructor or a reconstructed blockgenerator. The generated reconstructed signal may be used for the intraprediction of a next block to be processed within the current picture,and as described later, may also be output through filtering or may alsobe used for the inter prediction of a next picture.

Meanwhile, a luma mapping with chroma scaling (LMCS) may also be appliedin the picture decoding process.

The filter 350 may apply filtering to the reconstructed signal, therebyimproving the subjective/objective image qualities. For example, thefilter 350 may apply various filtering methods to the reconstructedpicture to generate a modified reconstructed picture, and transmit themodified reconstructed picture to the memory 360, specifically, the DPBof the memory 360. Various filtering methods may include, for example, adeblocking filtering, a sample adaptive offset, an adaptive loop filter,a bidirectional filter, and the like.

The (modified) reconstructed picture stored in the DPB of the memory 360may be used as the reference picture in the inter predictor 332. Thememory 360 may store motion information of the block in which the motioninformation within the current picture is derived (decoded) and/ormotion information of the blocks within the previously reconstructedpicture. The stored motion information may be transferred to the interpredictor 260 to be utilized as motion information of the spatialneighboring block or motion information of the temporal neighboringblock. The memory 360 may store the reconstructed samples of thereconstructed blocks within the current picture, and transfer the storedreconstructed samples to the intra predictor 331.

In the present specification, the exemplary embodiments described in thefilter 260, the inter predictor 221, and the intra predictor 222 of theencoding apparatus 200 may be applied equally to or to correspond to thefilter 350, the inter predictor 332, and the intra predictor 331 of thedecoding apparatus 300, respectively.

Meanwhile, as described above, in performing video coding, prediction isperformed to improve compression efficiency. Through this, a predictedblock including prediction samples for a current block as a block to becoded (i.e., a coding target block) may be generated. Here, thepredicted block includes prediction samples in a spatial domain (orpixel domain). The predicted block is derived in the same manner in anencoding apparatus and a decoding apparatus, and the encoding apparatusmay signal information (residual information) on residual between theoriginal block and the predicted block, rather than an original samplevalue of an original block, to the decoding apparatus, therebyincreasing image coding efficiency. The decoding apparatus may derive aresidual block including residual samples based on the residualinformation, add the residual block and the predicted block to generatereconstructed blocks including reconstructed samples, and generate areconstructed picture including the reconstructed blocks.

The residual information may be generated through a transform andquantization procedure. For example, the encoding apparatus may derive aresidual block between the original block and the predicted block,perform a transform procedure on residual samples (residual samplearray) included in the residual block to derive transform coefficients,perform a quantization procedure on the transform coefficients to derivequantized transform coefficients, and signal related residualinformation to the decoding apparatus (through a bit stream). Here, theresidual information may include value information of the quantizedtransform coefficients, location information, a transform technique, atransform kernel, a quantization parameter, and the like. The decodingapparatus may perform dequantization/inverse transform procedure basedon the residual information and derive residual samples (or residualblocks). The decoding apparatus may generate a reconstructed picturebased on the predicted block and the residual block. Also, for referencefor inter prediction of a picture afterward, the encoding apparatus mayalso dequantize/inverse-transform the quantized transform coefficientsto derive a residual block and generate a reconstructed picture basedthereon.

FIG. 4 is a control flowchart illustrating a video/image encoding methodto which the present disclosure may be applied.

S400 may be performed by the inter predictor 221 or the intra predictor222 of the encoding apparatus, and S410, S420, S430, and S440 may beperformed by the subtractor 231, the transformer 232, the quantizer 233,and the entropy encoder 240 of the encoding apparatus, respectively.

The encoding apparatus may derive prediction samples through predictionfor a current block (S400). The encoding apparatus may determine whetheror not to perform inter prediction or intra prediction on the currentblock, and determine a specific inter prediction mode or a specificintra prediction mode based on RD cost. According to the determinedmode, the encoding apparatus may derive prediction samples for thecurrent block.

The encoding apparatus may compare the prediction samples with originalsamples for the current block, and derive residual samples (S410).

The encoding apparatus derives transform coefficients through atransform process for the residual samples (S420). By quantizing thederived transform coefficients, quantized transform coefficients arederived (S430).

The encoding apparatus may encode image information including predictioninformation and residual information, and output the encoded imageinformation in the form of a bitstream (S440).

The prediction information may include an information on motioninformation (e.g., when the inter prediction is applied) and aprediction mode information as a plurality of informations related tothe prediction process. The residual information may be information onthe quantized transform coefficients, and, for example, includeinformation disclosed in Table 1 to be described later. The residualinformation may be entropy coded.

The output bitstream may be delivered to the decoding apparatus througha storage medium or a network.

FIG. 5 is a control flowchart illustrating a video/image decoding methodto which the present disclosure may be applied.

S500 may be performed by the inter predictor 332 or the intra predictor331 of the decoding apparatus. In S500, a process of decoding predictioninformation included in a bitstream and deriving values of relatedsyntax elements may be performed by the entropy decoder 310 of theencoding apparatus. S510, S520, S530, and S540 may be performed by theentropy decoder 310, the dequantizer 321, the inverse transformer 322,and the adder 340 of the decoding apparatus, respectively.

The decoding apparatus may perform an operation corresponding to theoperation which has been performed in the encoding apparatus. Thedecoding apparatus may perform inter prediction or intra prediction onthe current block and derive prediction samples, based on the receivedprediction information (S500).

The decoding apparatus may derive quantized transform coefficients forthe current block based on the received residual information (S510). Thedecoding apparatus may derive the quantized transform coefficients fromthe residual information through entropy decoding.

The decoding apparatus may dequantize the quantized transformcoefficients and derive transform coefficients (S520).

The decoding apparatus may derive residual samples through an inversetransform process for the transform coefficients (S530).

The decoding apparatus may generate the reconstructed samples for thecurrent block based on the residual samples and the prediction samples,and generate the reconstructed picture based on these reconstructedsamples. (S540).

As described above, after this, the in-loop filtering process may befurther applied to the reconstructed picture.

FIG. 6 shows a block diagram of context-adaptive binary arithmeticcoding (CABAC) for encoding a single syntax element, as a diagramillustrating a block diagram of a CABAC encoding system according to anembodiment.

In a case where an input signal is a non-binarized syntax element, anencoding process of the CABAC first converts the input signal into abinarized value through binarization. In a case where an input signal isalready a binarized value, the input signal bypasses the binarizationwithout being subject to it, and input to an encoding engine. Here, eachbinary number 0 or 1 constituting the binary value is referred to as abin. For example, in a case where a binary string after the binarizationis ‘110’, each of 1, 1, and 0 is referred to as a bin. The bin(s) for asyntax element may be a value of the syntax element.

Binarized bins are input to a regular encoding engine or a bypassencoding engine.

The regular encoding engine assigns to a corresponding bin a contextmodel reflecting a probability value, and encodes the bin based on theassigned context model. After performing the encoding on each bin, theregular encoding engine may update a probability model for the bin. Thethus encoded bins are referred to as context-coded bins.

The bypass encoding engine omits a process of estimating a probabilityfor an input bin, and a process of updating the probability model whichhas been applied to the bin, after the encoding. The bypass encodingengine improves an encoding speed by encoding bins being input theretowhile applying uniform probability distribution instead of assigning acontext. The thus encoded bins are referred to as bypass bins.

The entropy encoding may determine whether to perform the encodingthrough the regular encoding engine or through the bypass encodingengine, and switch an encoding path. The entropy decoding performs thesame processes as those of the encoding in a reverse order.

Meanwhile, in an embodiment, a (quantized) transform coefficient isencoded and/or decoded based on syntax elements, such astransform_skip_flag, last_sig_coeff_x_prefix, last_sig_coeff_y_prefix,last_sig_coeff_x_suffix, last_sig_coeff_y_suffix, coded_sub_block_flag,sig_coeff_flag, par_level_flag, rem_abs_gt1_flag, rem_abs_gt2_flag,abs_remainder, coeff_sign_flag, mts_idx and the like. Table 1 belowshows syntax elements related to the residual data encoding according toan example.

TABLE 1 Descriptor residual_coding( x0, y0, log2TbWidth, log2TbHeight,cIdx ) {  if( transform_skip_enabled_flag && ( cIdx ! = 0 ||cu_mts_flag[ x0 ][ y0 ] = = 0 ) &&   ( log2TbWidth <= 2 ) && (log2TbHeight <= 2 ) )   transform_skip_flag[ x0 ][ y0 ][ cIdx ] ae(v) last_sig_coeff_x_prefix ae(v)  last_sig_coeff_y_prefix ae(v)  if(last_sig_coeff_x_prefix > 3 )   last_sig_coeff_x_suffix ae(v)  if(last_sig_coeff_y_prefix > 3 )   last_sig_coeff_y_suffix ae(v) log2SbSize = ( Min( log2TbWidth, log2TbHeight ) < 2 ? 1 : 2 ) numSbCoeff = 1 << ( log2SbSize << 1 )  lastScanPos = numSbCoeff lastSubBlock = ( 1 << ( log2TbWidth + log2TbHeight − 2 * log2SbSize ) )− 1  do {   if( lastScanPos = = 0 ) {    lastScanPos = numSbCoeff   lastSubBlock− −   }   lastScanPos− −   xS = DiagScanOrder[log2TbWidth − log2SbSize ][ log2TbHeight − log2SbSize ]         [lastSubBlock ][ 0 ]   yS = DiagScanOrder[ log2TbWidth − log2SbSize ][log2TbHeight − log2SbSize ]         [ lastSubBlock ][ 1 ]   xC = ( xS <<log2SbSize ) +     DiagScanOrder[ log2SbSize ][ log2SbSize ][lastScanPos ][ 0 ]   yC = ( yS << log2SbSize ) +     DiagScanOrder[log2SbSize ][ log2SbSize ][ lastScanPos ][ 1 ]  } while( ( xC !=LastSignificantCoeffX ) || ( yC != LastSignificantCoeffY ) )  QState = 0 for( i = lastSubBlock; i >= 0; i− − ) {   startQStateSb = QState   xS =DiagScanOrder[ log2TbWidth − log2SbSize ][ log2TbHeight − log2SbSize ]        [ lastSubBlock ][ 0 ]   yS = DiagScanOrder[ log2TbWidth −log2SbSize ][ log2TbHeight − log2SbSize ]         [ lastSubBlock ][ 1 ]  inferSbDcSigCoeffFlag = 0   if( ( i < lastSubBlock ) && ( i > 0 ) ) {   coded_sub_block_flag[ xS ][ yS ] ae(v)    inferSbDcSigCoeffFlag = 1  }   firstSigScanPosSb = numSbCoeff   lastSigScanPosSb = −1   for( n =( i = = lastSubBlock ) ? lastScanPos −1 : numSbCoeff − 1; n >= 0; n− − ){    xC = ( xS << log2SbSize ) + DiagScanOrder[ log2SbSize ][ log2SbSize][ n ][ 0 ]    yC = ( yS << log2SbSize ) + DiagScanOrder[ log2SbSize ][log2SbSize ][ n ][ 1 ]    if( coded_sub_block_flag[ xS ][ yS ] && ( n >0 || !inferSbDcSigCoeffFlag ) ) {     sig_coeff_flag[ xC ][ yC ] ae(v)   }    if( sig_coeff_flag[ xC ][ yC ] ) {     par_level_flag[ n ] ae(v)    rem_abs_gt1_flag[ n ] ae(v)     if( lastSigScanPosSb = = −1 )     lastSigScanPosSb = n     firstSigScanPosSb = n    }   AbsLevelPass1[ xC ][ yC ] =      sig_coeff_flag[ xC ][ yC ] +par_level_flag[ n ] + 2 * rem_abs_gt1_flag[ n ]    if(dep_quant_enabled_flag )     QState = QStateTransTable[ QState ][par_level_flag[ n ] ]   }   for( n = numSbCoeff − 1; n >= 0; n− − ) {   if( rem_abs_gt1_flag[ n ] )     rem_abs_gt2_flag[ n ] ae(v)   }  for( n = numSbCoeff − 1; n >= 0; n− − ) {    xC = ( xS << log2SbSize) + DiagScanOrder[ log2SbSize ][ log2SbSize ][ n ][ 0 ]    yC = ( yS <<log2SbSize ) + DiagScanOrder[ log2SbSize ][ log2SbSize ][ n ][ 1 ]   if( rem_abs_gt2_flag[ n ] )     abs_remainder[ n ]    AbsLevel[ xC ][yC ] = AbsLevelPass1[ xC ][ yC ] +           2 * ( rem_abs_gt2_flag[ n] + abs_remainder[ n ] )   }   if( dep_quant_enabled_flag ||!sign_data_hiding_enabled_flag )    signHidden = 0   else    signHidden= ( lastSigScanPosSb − firstSigScanPosSb > 3 ? 1 : 0 )   for( n =numSbCoeff − 1; n >= 0; n− − ) {    xC = ( xS << log2SbSize ) +DiagScanOrder[ log2SbSize ][ log2SbSize ][ n ][ 0 ]    yC = ( yS <<log2SbSize ) + DiagScanOrder[ log2SbSize ][ log2SbSize ][ n ][ 1 ]   if( sig_coeff_flag[ xC ][ yC ] &&     ( !signHidden || ( n !=firstSigScanPosSb ) ) )     coeff_sign_flag[ n ] ae(v)   }   if(dep_quant_enabled_flag ) {    QState = startQStateSb    for( n =numSbCoeff − 1; n >= 0; n− − ) {     xC = ( xS << log2SbSize ) +      DiagScanOrder[ log2SbSize ][ log2SbSize ][ n ][ 0 ]      yC = ( yS<< log2SbSize ) +        DiagScanOrder[ log2SbSize ][ log2SbSize ][ n ][1 ]      if( sig_coeff_flag[ xC ][ yC ] )       TransCoeffLevel[ x0 ][y0 ][ cIdx ][ xC ][ yC ] =         ( 2 * AbsLevel[ xC ][ yC ] − (QState > 1 ? 1 : 0 ) ) *         ( 1 − 2 * coeff_sign_flag[ n ] )     QState = QStateTransTable[ QState ][ par_level_flag[ n ] ]    }else {     sumAbsLevel = 0     for( n = numSbCoeff − 1; n >= 0; n− − ) {     xC = ( xS << log2SbSize ) +        DiagScanOrder[ log2SbSize ][log2SbSize ][ n ][ 0 ]      yC = ( yS << log2SbSize ) +       DiagScanOrder[ log2SbSize ][ log2SbSize ][ n ][ 1 ]      if(sig_coeff_flag[ xC ][ yC ] ) {       TransCoeffLevel[ x0 ][ y0 ][ cIdx][ xC ][ yC ] =         AbsLevel[ xC ][ yC ] * ( 1 − 2 *coeff_sign_flag[ n ] )       if( signHidden ) {        sumAbsLevel +=AbsLevel[ xC][ yC ]        if( ( n = = firstSigScanPosSb ) && (sumAbsLevel % 2 ) = = 1 ) )         TransCoeffLevel[ x0 ][ y0 ][ cIdx ][xC ][ yC ] =           −TransCoeffLevel[ x0 ][ y0 ][ cIdx ][ xC ][ yC ]      }      }     }    }   }   if( cu_mts_flag[ x0 ][ y0 ] && ( cIdx == 0 ) &&    !transform_skip_flag[ x0 ][ y0 ][ cIdx ] &&    ( (CuPredMode[ x0 ][ y0 ] = = MODE_INTRA && numSigCoeff > 2 ) ||     (CuPredMode[ x0 ][ y0 ] = = MODE_INTER ) ) ) {    mts_idx[ x0 ][ y0 ]ae(v)  }

transform_skip_flag indicates whether or not a transform for anassociated block is skipped. The associated block may be a coding block(CB) or a transform block (TB). In connection with the transform (andthe quantization) and the residual coding process, the CB and the TB maybe used interchangeably. For example, as described above, residualsamples for the CB may be derived, and (quantized) transformcoefficients may be derived through transform and quantization on theresidual samples. And through the residual coding process, information(e.g., syntax elements) efficiently indicating positions, sizes, signsor the like of the (quantized) transform coefficients may be generatedand signaled. The quantized transform coefficients may be simply calledtransform coefficients. Generally, in a case where the CB is not greaterthan a maximum TB, the size of the CB may be the same as that of the TB,and in this case, a target block to be transformed (and quantized) andresidual-coded may be called the CB or the TB. Meanwhile, in a casewhere the CB is greater than the maximum TB, the target block to betransformed (and quantized) and residual-coded may be called the TB.While, hereinafter, syntax elements related to residual coding aredescribed by way of example as being signaled in units of a transformblocks (TBs), the TB and the coding block (CB) may be usedinterchangeably as described above.

In one embodiment, based on the syntax elements last_sig_coeff_x_prefix,last_sig_coeff_y_prefix, last_sig_coeff_x_suffix, andlast_sig_coeff_y_suffix, the (x, y) position information of the lastnon-zero transform coefficient in the transform block may be encoded.More specifically, last_sig_coeff_x_prefix indicates a prefix of acolumn position of a last significant coefficient in a scanning order ina transform block; last_sig_coeff_y_prefix indicates a prefix of a rowposition of a last significant coefficient in the scanning order in thetransform block; last_sig_coeff_x_suffix indicates a suffix of a columnposition of a last significant coefficient in the scanning order in thetransform block; and last_sig_coeff_y_suffix indicates a suffix of a rowposition of a last significant coefficient in the scanning order in thetransform block. Here, the significant coefficient may be the non-zerocoefficient. The scanning order may be a right upward diagonal scanningorder. Alternatively, the scanning order may be a horizontal scanningorder, or a vertical scanning order. The scanning order may bedetermined based on whether or not the intra/inter prediction is appliedto a target block (CB, or CB including TB), and/or a specificintra/inter prediction mode.

Next, after dividing a transform block into 4×4 sub-blocks, a one-bitsyntax element for coded_sub_block_flag, may used for each 4×4 sub-blockto indicate whether or not there is a non-zero coefficient in a currentsub-block.

If the value of coded_sub_block_flag is 0, there is no more informationto be transmitted, and therefore, the encoding process for the currentsub-block may be terminated. Conversely, if the value ofcoded_sub_block_flag is 1, the encoding process for sig_coeff_flag maycontinue to be performed. Since the sub-block including the lastnon-zero coefficient does not require encoding of coded_sub_block_flag,and the sub-block including the DC information of the transform blockhas a high probability of including the non-zero coefficient,coded_sub_block_flag may be assumed to have a value of 1 without beingencoded.

If it is determined that a non-zero coefficient exists in the currentsub-block because the value of coded_sub_block_flag is 1, then,inversely, sig_coeff_flag having a binary value may be encoded accordingto the scan order. A 1-bit syntax element sig_coeff_flag may be encodedfor each coefficient according to the scan order. If the value of thetransform coefficient at the current scan position is not 0, the valueof sig_coeff_flag may be 1. Here, in the case of a sub-block includingthe last non-zero coefficient, since sig_coeff_flag is not required tobe encoded for the last non-zero coefficient, the encoding process forsig_coeff_flag may be omitted. Only when sig_coeff_flag is 1, levelinformation encoding may be performed, and four syntax elements may beused in the level information encoding process. More specifically, eachsig_coeff_flag[xC][yC] may indicate whether or not the level (value) ofthe corresponding transform coefficient at each transform coefficientposition (xC, yC) in the current TB is non-zero. In an embodiment, thesig_coeff_flag may correspond to an example of a significant coefficientflag indicating whether or not a quantized transform coefficient is anon-zero effective coefficient.

The level value remaining after the encoding for sig_coeff_flag may bethe same as in Equation 1 below. That is, the syntax element remAbsLevelindicating the level value to be encoded may be as shown in Equation 1below. Here, coeff means an actual transform coefficient value.remAbsLevel=|coeff|−1  [Equation 1]

Through par_level_flag, the least significant coefficient (LSB) value ofremAbsLevel written in Equation 1 may be encoded as shown in Equation 2below. Here, par_level_flag[n] may indicate a parity of a transformcoefficient level (value) at a scanning position n. After par_leve_flagencoding, a transform coefficient level value remAbsLevel to be encodedmay be updated as shown in Equation 3 below.par_level_flag=remAbsLevel & 1  [Equation 2]remAbsLevel′=remAbsLevel>>1  [Equation 3]

rem_abs_gt1_flag may indicate whether or not remAbsLevel′ at thecorresponding scanning position n is greater than 1, andrem_abs_gt2_flag may indicate whether or not remAbsLevel′ at thecorresponding scanning position n is greater than 2. Encoding forabs_remainder may be performed only when rem_abs_gt2_flag is 1. When therelationship between the actual transform coefficient value coeff andeach syntax element is summarized, it may be, for example, as inEquation 4 below, and Table 2 below shows examples related to Equation4. In addition, the sign of each coefficient may be encoded using a1-bit symbol coeff_sign_flag. |coeff| may indicate a transformcoefficient level (value), and may be expressed as AbsLevel for atransform coefficient.|coeff|=sig_coeff_flag+par_level_flag+2*(rem_abs_gt1_flag+rem_abs_gt2_flag+abs_remainder)  [Equation4]

In an embodiment, the par_level_flag indicates an example of a paritylevel flag for parity of a transform coefficient level for the quantizedtransform coefficient, the rem_abs_gt1_flag indicates an example of afirst transform coefficient level flag for whether or not the transformcoefficient level is greater than a first threshold value, and therem_abs_gt2_flag may indicate an example of a second transformcoefficient level flag for whether or not the transform coefficientlevel is greater than a second threshold value.

In addition, in another embodiment, rem_abs_gt2_flag may be referred toas rem_abs_gt3_flag, and in another embodiment, rem_abs_gt1_flag andrem_abs_gt2_flag may be represented based on abs_level_gtx_flag[n][j].abs_level_gtx_flag[n][j] may be a flag indicating whether or not theabsolute value of the transform coefficient level at the scanningposition n (or the transform coefficient level shifted by 1 to theright) is greater than (j<<1)+1. In one example, the rem_abs_gt1_flagmay perform a function which is the same and/or similar function toabs_level_gtx_flag[n][0], and the rem_abs_gt2_flag may perform afunction which is the same and/or similar to abs_level_gtx_flag[n][1].That is, the abs_level_gtx_flag[n][0] may correspond to an example ofthe first transform coefficient level flag, and theabs_level_gtx_flag[n][1] may correspond to an example of the secondtransform coefficient level flag. The (j<<1)+1 may be replaced by apredetermined threshold value, such as a first threshold value and asecond threshold value, according to circumstances.

TABLE 2 [coeff] sig_coeff_flag par_level_flag rem_abs_gt1_flagrem_abs_gt2_flag abs_remainder 0 0 1 1 0 0 2 1 1 0 3 1 0 1 0 4 1 1 1 0 51 0 1 1 0 6 1 1 1 1 0 7 1 0 1 1 1 8 1 1 1 1 1 9 1 0 1 1 2 10 1 1 1 1 211 1 0 1 1 3 . . . . . . . . . . . . . . . . . .

FIG. 7 is a diagram illustrating an example of transform coefficients ina 4×4 block.

The 4×4 block of FIG. 7 shows an example of quantized coefficients. Theblock illustrated in FIG. 7 may be a 4×4 transform block, or a 4×4sub-block of an 8×8, 16×16, 32×32, or 64×64 transform block. The 4×4block of FIG. 7 may be a luminance block or a chrominance block. Theencoding result for the inverse diagonally scanned coefficients of FIG.7 may be, for example, shown in Table 3. In Table 3, scan_pos indicatesthe position of the coefficient according to the inverse diagonal scan.scan_pos 15 is a coefficient which is scanned first in the 4×4 block,that is, a coefficient at a bottom-right corner, and scan_pos 0 is acoefficient which is scanned last, that is, a coefficient at a top-leftcorner. Meanwhile, in one embodiment, the scan_pos may be referred to asa scan position. For example, the scan_pos 0 may be referred to as scanposition 0.

TABLE 3 scan_pos 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 coefficients 0 00 0 1 −1 0 2 0 3 −2 −3 4 6 −7 10 sig_coeff_flag 0 0 0 0 1 1 0 1 0 1 1 11 1 1 1 par_level_flag 0 0 1 0 1 0 1 1 0 1 rem_abs_gt1_flag 0 0 0 1 0 11 1 1 1 rem_abs_gt2_flag 0 0 0 1 1 1 abs_remainder 0 1 2 ceoff_sign_flag0 1 0 0 1 1 0 0 1 0

Meanwhile, as described with reference to Table 1, prior to encoding theresidual signal and the special residual signal, whether or not to applythe transform of the corresponding block is first transmitted. Byexpressing the correlation between the residual signals in the transformdomain, compaction of data is achieved and transmitted to the decodingapparatus. If the correlation between the residual signals isinsufficient, data compaction may not occur sufficiently. In this case,a conversion process including a complex calculation process may beomitted, and a residual signal in the pixel domain (spatial domain) maybe transferred to the decoding apparatus.

Since the residual signal of the pixel domain that has not beensubjected to transform has different characteristics (the distributionof the residual signal, the absolute level of each residual signal,etc.) from the residual signal of the general transformation domain, aresidual signal encoding method for efficiently transmitting such asignal to a decoding apparatus according to an example of the presentdisclosure will be proposed hereinafter.

FIG. 8 is a diagram illustrating a residual signal decoder according toan example of the present disclosure.

As illustrated, a transformation application flag indicating whether ornot a transform is applied to a corresponding transform block andinformation on the encoded binarization code may be input to theresidual signal decoder 800, and a decoded residual signal may be outputfrom the residual signal decoder 800.

A flag for whether or not to apply a transform may be expressed astransform_skip_flag, and the encoded binarization code may be input tothe residual signal decoder 800 through the binarization process throughFIG. 6 .

The transform skip flag is transmitted in units of transform blocks, andin Table 1 the flag for whether or not to transform is limited to aspecific block size (a condition of parsing the transform skip flag isincluded only when the transform block size is 4×4 or less). However, inthe present embodiment, the size of a block for determining whether ornot to parse the transform skip flag may be variously configured. Thesizes of Log 2TbWidth and log 2TbHeight are determined as variables wNand hN, and the wN and the hN may be selected as one of the following.wN={2,3,4,5}wH={2,3,4,5}  [Equation 5]

A syntax element to which Equation 5 may be applied is as follows.

TABLE 4  if( transform_skip_enabled_flag && ( cIdx ! = 0 || cu_mts_flag[x0 ][ y0 ] = = 0 ) &&   ( log2TbWidth <= wN ) && ( log2TbHeight <= hN ))   transform_skip_flag[ x0 ][ y0 ][ cIdx ] ae(v)

As described above, a method of decoding the residual signal may bedetermined according to the transform skip flag. Through the proposedmethod, it is possible to reduce the complexity in the entropy decodingprocess and improve the encoding efficiency by efficiently processingsignals having different statistical characteristics from each other.

Based on the description in Table 1 and the above embodiment, in thisembodiment, when the current decoding target block is residuals of theuntransformed pixel domain, a method of decoding a residual signalencoded in a 4×4 sub-block unit in units of transform blocks (TBs) isproposed.

The general residual signal is expressed as a transform domain, and asthe transformed residual signal gets closer to the top-left based on theblock's coefficient, a non-zero coefficient is more likely to occur, andthe absolute value level of the coefficient may also have a greatervalue. The encoding may be performed by the above-described method,reflecting these characteristics.

However, the residual of the pixel domain that is not expressed in thetransform domain does not have the above characteristics, and theprobability of generating a coefficient of zero or more has randomness.In this case, the method of determining an element to be encoded ascoded_sub_block_flag, in a unit higher than the pixel unit by expressingthe residual in units of sub-blocks may rather cause a side effect ofredundantly transmitting information on the coefficient distribution,that is, an increase in complexity. Accordingly, according to thepresent embodiment, for a transform block to which a transform is notapplied, encoding and decoding efficiency may be improved bytransmitting a residual signal in a transform block unit instead of asub-block unit.

This is summarized with reference to FIG. 9 as follows. FIG. 9 is acontrol flowchart illustrating a method of decoding a residual signalaccording to an exemplary embodiment of the present disclosure.

First, the entropy decoder or the residual signal decoder of thedecoding apparatus parses a transform skip flag (transform_skip_flag)indicating whether or not a transform process has been performed on atransform block (S900), and it may be determined based on the parsedinformation whether a residual signal has been transformed (S910).

As a result of the determination, when the transform skip flag indicatesthat the residual signal has been transformed, the entropy decoder orthe residual signal decoder may decode the transformed block in units ofsub-blocks (S920).

Contrarily, when the transform skip flag indicates that the residualsignal has not been transformed, the entropy decoder or the residualsignal decoder may decode the transform block in units of transformblocks rather than in units of sub-blocks (S930).

Meanwhile, based on the technique of decoding the residual based onTable 1 and the transform skip flag, the present embodiment proposes amethod of determining a context element, that is, a syntax, when thecurrent decoding target block is a residual of an untransformed pixeldomain.

In the case of a general transform domain residual, the residual signalis expressed as a level value for each frequency component, and in thehigh frequency region, the probability of being expressed as zero or anumber close to zero by quantization increases. Therefore, in Table 1, amethod was used in which the subsequent context element parsing may beomitted by first encoding sig_coeff_flag, which is a context element forwhether or not the current transform coefficient value is 0.

When sig_coeff_flag is not 0, rem_abs_gt1_flag, par_level_flag,rem_abs_gt2_flag, and the like may be sequentially encoded according tothe value of the current transform coefficient. However, in the case ofa residual signal of a pixel domain that has not been subjected to thetransform, the absolute level value of the signal has randomness.

The context-encoded syntax element may include at least one of thesig_coeff_flag, par_level_flag, rem_abs_gt1_flag, and/orrem_abs_gt2_flag as a syntax element that is encoded through arithmeticcoding based on context. In addition, hereinafter, the context encodingbin may indicate a context-encoded bin for at least one of saidsig_coeff_flag, par_level_flag, rem_abs_gt1_flag, and/orrem_abs_gt2_flag.

In general, in a case where the value of the residual signal is large,when all of the syntax elements, such as sig_coeff_flag, par_level_flag,rem_abs_gt1_flag, and _rem_abs_gt2_flag, are expressed in allcoefficients, redundant information is more likely to be transmittedcompared to transmitting the level value by binarizing it as it is.Accordingly, the present embodiment proposes a method of improvingencoding efficiency by omitting some context elements for the residualsignal of the pixel domain.

The proposed method may be branched based on the transform_skip_flagcontext element in Table 1, and, the existing method that does notcorrespond to the branch statement may follow, for example, the contextelement (sig_coeff_flag, par_level_flag, rem_abs_gt1_flag,rem_abs_gt2_flag, abs_remainder, coeff_sign_flag) of Table 1, or may beencoded and decoded including the context element defined above. Thatis, when the transform is applied, context elements of sig_coeff_flag,par_level_flag, rem_abs_gt1_flag, rem_abs_gt2_flag, abs_remainder, andcoeff_sign_flag may be encoded and decoded as shown in Table 1.

Meanwhile, the residual signal to which the transforma is not appliedmay be encoded and decoded through context elements of sig_coeff_flag,par_level_flag, rem_abs_gt1_flag, abs_remainder, and coeff_sign_flag.

Table 5 shows the context elements according to the present embodiment.

TABLE 5 Descriptor residual_coding( x0, y0, log2TbWidth, log2TbHeight,cIdx ) {  if( transform_skip_enabled_flag && ( cIdx ! = 0 ||cu_mts_flag[ x0 ][ y0 ] = = 0 ) &&   ( log2TbWidth <= wN ) && (log2TbHeight <= hN ) )   transform_skip_flag[ x0 ][ y0 ][ cIdx ] ae(v)...  if ( transform_skip_flag[x0][y0][cIdx] )  {   for( n = ( i = =lastSubBlock ) ? lastScanPos − 1 : numSbCoeff − 1; n >= 0; n− − ) {   xC = ( xS << log2SbSize ) + DiagScanOrder[ log2SbSize ][ log2SbSize][ n ][ 0 ]    yC = ( yS << log2SbSize ) + DiagScanOrder[ log2SbSize ][log2SbSize ][ n ][ 1 ]    if( coded_sub_block_flag[ xS ][ yS ] && ( n >0 || !inferSbDcSigCoeffFlag ) ) {     sig_coeff_flag[ xC ][ yC ] ae(v)   }    if( sig_coeff_flag[ xC ][ yC ] ) {     par_level_flag[ n ] ae(v)    rem_abs_gt1_flag[ n ] ae(v)     if( lastSigScanPosSb = = −1 )     lastSigScanPosSb = n     firstSigScanPosSb = n    }   AbsLevelPass1[ xC ][ yC ] =      sig_coeff_flag[ xC][ yC ] +par_level_flag[ n ] + 2 * rem_abs_gt1_flag[ n ]    if(dep_quant_enabled_flag )     QState = QStateTransTable[ QState ][par_level_flag[ n ] ]   }   for( n = numSbCoeff − 1; n >= 0; n− − ) {   xC = ( xS << log2SbSize ) + DiagScanOrder[ log2SbSize ][ log2SbSize][ n ][ 0 ]    yC = ( yS << log2SbSize ) + DiagScanOrder[ log2SbSize ][log2SbSize ][ n ][ 1 ]    if( rem_abs_gt1_flag[ n ] )     abs_remainder[n ]    AbsLevel[ xC ][ yC ] = AbsLevelPass1[ xC ][ yC ] +           2 *( abs_remainder[ n ] )   }   if( dep_quant_enabled_flag ||!sign_data_hiding_enabled_flag )    signHidden = 0   else    signHidden= ( lastSigScanPosSb − firstSigScanPosSb > 3 ? 1 : 0 )   for( n =numSbCoeff − 1; n >= 0; n− − ) {    xC = ( xS << log2SbSize ) +DiagScanOrder[ log2SbSize ][ log2SbSize ][ n ][ 0 ]    yC = ( yS <<log2SbSize ) + DiagScanOrder[ log2SbSize ][ log2SbSize ][ n ][ 1 ]   if( sig_coeff_flag[ xC ][ yC ] &&     ( !signHidden || ( n !=firstSigScanPosSb ) ) )     coeff_sign_flag[ n ] ae(v)   }   if(dep_quant_enabled_flag ) {    QState = startQStateSb    for( n =numSbCoeff − 1; n >= 0; n− − ) {     xC = ( xS << log2SbSize ) +      DiagScanOrder[ log2SbSize ][ log2SbSize ][ n ][ 0 ]     yC = ( yS<< log2SbSize ) +       DiagScanOrder[ log2SbSize ][ log2SbSize ][ n ][1 ]     if( sig_coeff_flag[ xC ][ yC ] )      TransCoeffLevel[ x0 ][ y0][ cIdx ][ xC ][ yC ] =        ( 2 * AbsLevel[ xC ][ yC ] − ( QState > 1? 1 : 0 ) ) *        ( 1 − 2 * coeff_sign_flag[ n ] )     QState =QStateTransTable[ QState ][ par_level_flag[ n ] ]   } else {   sumAbsLevel = 0    for( n = numSbCoeff − 1; n >= 0; n− − ) {     xC =( xS << log2SbSize ) +       DiagScanOrder[ log2SbSize ][ log2SbSize ][n ][ 0 ]     yC = ( yS << log2SbSize ) +       DiagScanOrder[ log2SbSize][ log2SbSize ][ n ][ 1 ]     if( sig_coeff_flag[ xC ][ yC ] ) {     TransCoeffLevel[ x0 ][ y0 ][ cIdx ][ xC ][ yC ] =        AbsLevel[xC ][ yC ] * ( 1 − 2 * coeff_sign_flag[ n ] )      if( signHidden ) {      sumAbsLevel += AbsLevel[ xC][ yC ]       if( ( n = =firstSigScanPosSb ) && ( sumAbsLevel % 2 ) = = 1 ) )       TransCoeffLevel[ x0 ][ y0 ][ cIdx ][ xC ][ yC ] =         −TransCoeffLevel[ x0 ][ y0 ][ cIdx ][ xC ][ yC ]      }     }   }   }  }  }  else { //transform_skip_flag  ...  }  if( cu_mts_flag[x0 ][ y0 ] && ( cIdx = = 0 ) &&   !transform_skip_flag[ x0 ][ y0 ][ cIdx] &&   ( ( CuPredMode [ x0 ][ y0 ] = = MODE_INTRA && numSigCoeff > 2 )||    ( CuPredMode[ x0 ][ y0 ] = = MODE_INTER ) ) ) {   mts_idx[ x0 ][y0 ] ae(v) }

FIG. 10 is a control flowchart illustrating a process of parsing acontext element according to the present embodiment. The context elementparsing according to the transform skip flag according to FIG. 10 willbe described as follows.

First, a transform skip flag (transform_skip_flag) indicating whether atransform process has been performed on a transform block is parsed todetermine whether or not transform_skip_flag is 1 (S1000).

As a result of the determination, in the case of the residual valuewhose transform_skip_flag is 1, that is, for which the transform hasbeen skipped without being applied, the context elements ofsig_coeff_flag, par_level_flag, rem_abs_gt1_flag, abs_remainder, andcoeff_sign_flag may be encoded, and the context elements ofsig_coeff_flag, par_level_flag, rem_abs_gt1_flag, abs_remainder, andcoeff_sign_flag may be parsed (S1010).

In this case, the context elements may be sequentially parsed or theparsing order may be changed.

Contrarily, in the case of the residual value whose transform_skip_flagis 0, that is, to which the transform has been applied, the contextelements of sig_coeff_flag, par_level_flag, rem_abs_gt1_flag,rem_abs_gt2_flag, abs_remainder, and coeff_sign_flag may be encoded, andthe context elements of sig_coeff_flag, par_level_flag,rem_abs_gt1_flag, rem_abs_gt2_flag, abs_remainder, and coeff_sign_flagmay be parsed (S1020). In this case, the context elements may besequentially parsed or the parsing order may be changed.

That is, in the case of the residual value to which the transform is notapplied, rem_abs_gt2_flag is not encoded and decoded, when compared withthe residual value to which the transform is applied. In a case wherethe residual value is large, when all of the syntax elements, such assig_coeff_flag, par_level_flag, rem_abs_gt1_flag and _rem_abs_gt2_flag,are expressed in all coefficients, redundant information is more likelyto be transmitted compared to transmitting the level value by binarizingit as it is, and thus, in the present embodiment, encoding efficiency isimproved by omitting the context element of rem_abs_gt2_flag.

Meanwhile, based on the technique of decoding the residual based onTable 1 and the transform skip flag, another embodiment according to thepresent disclosure proposes a method of determining a context element,that is, a syntax, when the current decoding target block is a residualof an untransformed pixel domain.

In the case of a general transform domain residual, the residual signalis expressed as a level value for each frequency component, and in thehigh frequency region, the probability of being expressed as zero or anumber close to zero by quantization increases. Therefore, in Table 1, amethod was used in which the subsequent context element parsing may beomitted by first encoding sig_coeff_flag, which is a context element forwhether or not the current transform coefficient value is 0.

When sig_coeff_flag is not 0, rem_abs_gt1_flag, par_level_flag,rem_abs_gt2_flag, and the like may be sequentially encoded according tothe value of the current transform coefficient. However, in the case ofa residual signal of a pixel domain that has not been subjected to thetransform, the absolute level value of the signal has randomness.

The context-encoded syntax element may include at least one of thesig_coeff_flag, par_level_flag, rem_abs_gt1_flag, and/orrem_abs_gt2_flag as a syntax element that is encoded through arithmeticcoding based on context. In addition, hereinafter, the context encodingbin may indicate a context-encoded bin for at least one of saidsig_coeff_flag, par_level_flag, rem_abs_gt1_flag, and/orrem_abs_gt2_flag.

In general, in a case where the value of the residual signal is large,when all of the syntax elements, such as sig_coeff_flag, par_level_flag,rem_abs_gt1_flag, and _rem_abs_gt2_flag, are expressed in allcoefficients, redundant information is more likely to be transmittedcompared to transmitting the level value by binarizing it as it is.Accordingly, the present embodiment proposes a method of improvingencoding efficiency by omitting some context elements for the residualsignal of the pixel domain.

The proposed method may be branched based on the transform_skip_flagcontext element in Table 1, and, the existing method that does notcorrespond to the branch statement may follow, for example, the contextelement (sig_coeff_flag, par_level_flag, rem_abs_gt1_flag,rem_abs_gt2_flag, abs_remainder, coeff_sign_flag) of Table 1, or may beencoded and decoded including the context element defined above. Thatis, when the transform is applied, context elements of sig_coeff_flag,par_level_flag, rem_abs_gt1_flag, rem_abs_gt2_flag, abs_remainder, andcoeff_sign_flag may be encoded and decoded as shown in Table 1.

Meanwhile, the residual signal to which the transforma has not beenapplied may be encoded and decoded through context elements ofsig_coeff_flag, par_level_flag, abs_remainder, and coeff_sign_flag.

Table 6 shows the context elements according to the present embodiment.

TABLE 6 Descriptor residual_coding( x0, y0, log2TbWidth, log2TbHeight,cIdx ) {  if( transform_skip_enabled_flag && ( cIdx ! = 0 ||cu_mts_flag[ x0 ][ y0 ] = = 0 ) &&   ( log2TbWidth <= wN ) && (log2TbHeight <= hN ) )   transform_skip_flag[ x0 ][ y0 ][ cIdx ] ae(v)...  if ( transform_skip_flag[x0][y0][cIdx] )  {   for( n = ( i = =lastSubBlock ) ? lastScanPos − 1 : numSbCoeff − 1; n >= 0; n− − ) {   xC = ( xS << log2SbSize ) + DiagScanOrder[ log2SbSize ][ log2SbSize][ n ][ 0 ]    yC = ( yS << log2SbSize ) + DiagScanOrder[ log2SbSize ][log2SbSize ][ n ][ 1 ]    if( coded_sub_block_flag[ xS ][ yS ] && ( n >0 || !inferSbDcSigCoeffFlag ) ) {     sig_coeff_flag[ xC ][ yC ] ae(v)   }    if( sig_coeff_flag[ xC ][ yC ] ) {     par_level_flag[ n ] ae(v)    if( lastSigScanPosSb = = −1 )      lastSigScanPosSb = n    firstSigScanPosSb = n    }    AbsLevelPass1[ xC ][ yC ] =     sig_coeff_flag[ xC][ yC ] + par_level_flag[ n ]     if(dep_quant_enabled_flag )      QState = QStateTransTable[ QState ][par_level_flag[ n ] ]    }    for( n = numSbCoeff − 1; n >= 0; n− − ) {    xC = ( xS << log2SbSize ) +  DiagScanOrder[ log2SbSize ][ log2SbSize][ n ][ 0 ]     yC = ( yS << log2SbSize ) +  DiagScanOrder[ log2SbSize][ log2SbSize ][ n ][ 1 ]     if( sig_coeff_flag[ n ] )     abs_remainder[ n ]     AbsLevel[ xC ][ yC ] = AbsLevelPass1[ xC ][yC ] +           2 * ( abs_remainder[ n ] )    }    if(dep_quant_enabled_flag || !sign_data_hiding_enabled_flag )    signHidden = 0    else     signHidden = ( lastSigScanPosSb −firstSigScanPosSb > 3 ? 1 : 0 )    for( n = numSbCoeff − 1; n >= 0; n− −) {     xC = ( xS << log2SbSize ) +  DiagScanOrder[ log2SbSize ][log2SbSize ][ n ][ 0 ]     yC = ( yS << log2SbSize ) +  DiagScanOrder[log2SbSize ][ log2SbSize ][ n ][ 1 ]     if( sig_coeff_flag[ xC ][ yC ]&&      ( !signHidden || ( n != firstSigScanPosSb ) ) )     coeff_sign_flag[ n ] ae(v)    }    if( dep_quant_enabled_flag ) {    QState = startQStateSb     for( n = numSbCoeff − 1; n >= 0; n− − ) {     xC = ( xS << log2SbSize ) +       DiagScanOrder[ log2SbSize ][log2SbSize ][ n ][ 0 ]      yC = ( yS << log2SbSize ) +      DiagScanOrder[ log2SbSize ][ log2SbSize ][ n ][ 1 ]      if(sig_coeff_flag[ xC ][ yC ] )       TransCoeffLevel[ x0 ][ y0 ][ cIdx ][xC ][ yC ] =        ( 2 * AbsLevel[ xC ][ yC ] − ( QState > 1 ? 1 : 0 )) *        ( 1 − 2 * coeff_sign_flag[ n ] )      QState =QStateTransTable[ QState ][ par_level_flag[ n ] ]    } else {    sumAbsLevel = 0     for( n = numSbCoeff − 1; n >= 0; n− − ) {     xC = ( xS << log2SbSize ) +       DiagScanOrder[ log2SbSize ][log2SbSize ][ n ][ 0 ]      yC = ( yS << log2SbSize ) +      DiagScanOrder[ log2SbSize ][ log2SbSize ][ n ][ 1 ]      if(sig_coeff_flag[ xC ][ yC ] ) {       TransCoeffLevel[ x0 ][ y0 ][ cIdx][ xC ][ yC ] =        AbsLevel[ xC ][ yC ] * ( 1 − 2 * coeff_sign_flag[n ] )       if( signHidden ) {       sumAbsLevel += AbsLevel[ xC][ yC ]      if( ( n = = firstSigScanPosSb ) && ( sumAbsLevel % 2 ) = =  1 ) )       TransCoeffLevel[ x0 ][ y0 ][ cIdx ][ xC ][ yC ] =         −TransCoeffLevel[ x0 ][ y0 ][ cIdx ][ xC ][ yC ]       }      }    }    }   }   }   else {   }   if( cu_mts_flag[ x0 ][ y0 ] && ( cIdx= = 0 ) &&    !transform_skip_flag[ x0 ][ y0 ][ cIdx ] &&    ( (CuPredMode[ x0 ][ y0 ] = = MODE_INTRA && numSigCoeff > 2 ) ||     (CuPredMode[ x0 ][ y0 ] = = MODE_INTER ) ) ) {    mts_idx[ x0 ][ y0 ]ae(v) }

FIG. 11 is a control flowchart illustrating a process of parsing acontext element according to the present embodiment. The context elementparsing according to the transform skip flag according to FIG. 11 willbe described as follows.

First, a transform skip flag (transform_skip_flag) indicating whether atransform process has been performed on a transform block is parsed todetermine whether or not transform_skip_flag is 1 (S1100).

As a result of the determination, in the case of the residual valuewhose transform_skip_flag is 1, that is, for which the transform hasbeen skipped without being applied, the context elements ofsig_coeff_flag, par_level_flag, abs_remainder, and coeff_sign_flag maybe encoded, and the context elements of sig_coeff_flag, par_level_flag,abs_remainder, and coeff_sign_flag may be parsed (S1110). In this case,the context elements may be sequentially parsed or the parsing order maybe changed.

Contrarily, in the case of the residual value whose transform_skip_flagis 0, that is, to which the transform has been applied, the contextelements of sig_coeff_flag, par_level_flag, rem_abs_gt1_flag,rem_abs_gt2_flag, abs_remainder, and coeff_sign_flag may be encoded, andthe context elements of sig_coeff_flag, par_level_flag,rem_abs_gt1_flag, rem_abs_gt2_flag, abs_remainder, and coeff_sign_flagmay be parsed (S1120). In this case, the context elements may besequentially parsed or the parsing order may be changed.

That is, in the case of the residual value to which the transform hasnot been applied, rem_abs_gt1_flag and rem_abs_gt2_flag are not encodedand decoded, when compared with the residual value to which thetransform has been applied. In a case where the residual value is large,when all of the syntax elements, such as sig_coeff_flag, par_level_flag,rem_abs_gt1_flag and rem_abs_gt2_flag, are expressed in allcoefficients, redundant information is more likely to be transmittedcompared to transmitting the level value by binarizing it as it is, andthus, in the present embodiment, encoding efficiency is improved byomitting the context elements of rem_abs_gt1_flag and rem_abs_gt2_flag.

Meanwhile, based on the technique of decoding the residual based onTable 1 and the transform skip flag, still another embodiment accordingto the present disclosure proposes a method of determining a contextelement when the current decoding target block is a residual of anuntransformed pixel domain.

In the case of a general transform domain residual, the residual signalis expressed as a level value for each frequency component, and in thehigh frequency region, the probability of being expressed as zero or anumber close to zero by quantization increases. Therefore, in Table 1, amethod was used in which the subsequent context element parsing may beomitted by first encoding sig_coeff_flag, which is a context element forwhether or not the current transform coefficient value is 0.

When sig_coeff_flag is not 0, rem_abs_gt1_flag, par_level_flag,rem_abs_gt2_flag, and the like may be sequentially encoded according tothe value of the current transform coefficient. However, in the case ofa residual signal of a pixel domain that has not been subjected to thetransform, the absolute level value of the signal has randomness.

The context-encoded syntax element may include at least one of thesig_coeff_flag, par_level_flag, rem_abs_gt1_flag, and/orrem_abs_gt2_flag as a syntax element that is encoded through arithmeticcoding based on context. In addition, hereinafter, the context encodingbin may indicate a context-encoded bin for at least one of saidsig_coeff_flag, par_level_flag, rem_abs_gt1_flag, and/orrem_abs_gt2_flag.

In general, in a case where the value of the residual signal is large,when all of the syntax elements, such as sig_coeff_flag, par_level_flag,rem_abs_gt1_flag, and _rem_abs_gt2_flag, are expressed in allcoefficients, redundant information is more likely to be transmittedcompared to transmitting the level value by binarizing it as it is.Accordingly, the present embodiment proposes a method of improvingencoding efficiency by omitting some context elements for the residualsignal of the pixel domain.

The proposed method may be branched based on the transform_skip_flagcontext element in Table 1, and, the existing method that does notcorrespond to the branch statement may follow, for example, the contextelement (sig_coeff_flag, par_level_flag, rem_abs_gt1_flag,rem_abs_gt2_flag, abs_remainder, coeff_sign_flag) of Table 1, or may beencoded and decoded including the context element defined above. Thatis, when the transform is applied, context elements of sig_coeff_flag,par_level_flag, rem_abs_gt1_flag, rem_abs_gt2_flag, abs_remainder, andcoeff_sign_flag may be encoded and decoded as shown in Table 1.

Meanwhile, the residual signal to which the transforma has not beenapplied may be encoded and decoded through context elements ofsig_coeff_flag, abs_remainder, and coeff_sign_flag.

In addition, according to an example, when all the number of bin for thecontext encoding syntax element has been used, only abs_remainder andcoeff_sign_flag may be coded or decoded/parsed without coding ordecoding/parsing the context encoding syntax element any more.

Table 7 shows the context elements according to the present embodiment.

TABLE 7 Descriptor residual_coding( x0, y0, log2TbWidth, log2TbHeight,cIdx ) {  if( transform_skip_enabled_flag && ( cIdx ! = 0 ||cu_mts_flag[ x0 ][ y0 ] = = 0 ) &&   ( log2TbWidth <= wN ) && (log2TbHeight <= hN ) )   transform_skip_flag[ x0 ][ y0 ][ cIdx ] ae(v)...  if ( transform_skip_flag[x0][y0][cIdx] )  {   for( n = ( i = =lastSubBlock ) ? lastScanPos − 1 : numSbCoeff − 1; n >= 0; n− − ) {   xC = ( xS << log2SbSize ) + DiagScanOrder[ log2SbSize ][ log2SbSize][ n ][ 0 ]    yC = ( yS << log2SbSize ) + DiagScanOrder[ log2SbSize ][log2SbSize ][ n ][ 1 ]    if( coded_sub_block_flag[ xS ][ yS ] && ( n >0 || !inferSbDcSigCoeffFlag ) ) {     sig_coeff_flag[ xC ][ yC ] ae(v)   }    if( sig_coeff_flag[ xC ][ yC ] ) {     if( lastSigScanPosSb = =−1 )      lastSigScanPosSb = n     firstSigScanPosSb = n    }   AbsLevelPass1[ xC ][ yC ] =      sig_coeff_flag[ xC][ yC ]   for( n =numSbCoeff − 1; n >= 0; n− − ) {    xC = ( xS << log2SbSize ) +DiagScanOrder[ log2SbSize ][ log2SbSize ][ n ][ 0 ]    yC = ( yS <<log2SbSize ) + DiagScanOrder[ log2SbSize ][ log2SbSize ][ n ][ 1 ]    abs_remainder[ n ]    AbsLevel[ xC ][ yC ] = AbsLevelPass1[ xC ][ yC] +           abs_remainder[ n ]   }   if(!sign_data_hiding_enabled_flag)    signHidden = 0   else    signHidden = ( lastSigScanPosSb −firstSigScanPosSb > 3 ? 1 : 0 )   for( n = numSbCoeff − 1; n >= 0; n− −) {    xC = ( xS << log2SbSize ) + DiagScanOrder[ log2SbSize ][log2SbSize ][ n ][ 0 ]    yC = ( yS << log2SbSize ) + DiagScanOrder[log2SbSize ][ log2SbSize ][ n ][ 1 ]    if( sig_coeff_flag[ xC ][ yC ]&&     ( !signHidden || ( n != firstSigScanPosSb ) ) )    coeff_sign_flag[ n ] ae(v)   }    sumAbsLevel = 0    for( n =numSbCoeff − 1; n >= 0; n− − ) {     xC = ( xS << log2SbSize ) +      DiagScanOrder[ log2SbSize ][ log2SbSize ][ n ][ 0 ]     yC = ( yS<< log2SbSize ) +       DiagScanOrder[ log2SbSize ][ log2SbSize ][ n ][1 ]     if( sig_coeff_flag[ xC ][ yC ] ) {      TransCoeffLevel[ x0 ][y0 ][ cIdx ][ xC ][ yC ] =        AbsLevel[ xC ][ yC ] * ( 1 − 2 *coeff_sign_flag[ n ] )      if( signHidden ) {       sumAbsLevel +=AbsLevel[ xC][ yC ]       if( ( n = = firstSigScanPosSb ) && (sumAbsLevel % 2 ) = = 1 ) )        TransCoeffLevel[ x0 ][ y0 ][ cIdx ][xC ][ yC ] =          −TransCoeffLevel[ x0 ][ y0 ][ cIdx ][ xC ][ yC ]     }     }    }   }  }  else {  ...  }  if( cu_mts_flag[ x0 ][ y0 ] &&( cIdx = = 0 ) &&   !transform_skip_flag[ x0 ][ y0 ][ cIdx ] &&   ( (CuPredMode[ x0 ][ y0 ] = = MODE_INTRA && numSigCoeff > 2 ) ||    (CuPredMode[ x0 ][ y0 ] = = MODE_INTER ) ) ) {   mts_idx[ x0 ][ y0 ]ae(v) }

FIG. 12 is a control flowchart illustrating a process of parsing acontext element according to the present embodiment. The context elementparsing according to the transform skip flag according to FIG. 12 willbe described as follows.

First, a transform skip flag (transform_skip_flag) indicating whether atransform process has been performed on a transform block is parsed todetermine whether or not transform_skip_flag is 1 (S1200).

As a result of the determination, in the case of the residual valuewhose transform_skip_flag is 1, that is, for which the transform hasbeen skipped without being applied, the context elements ofsig_coeff_flag, abs_remainder, and coeff_sign_flag may be encoded, andthe context elements of sig_coeff_flag, abs_remainder, andcoeff_sign_flag may be parsed (S1210). In this case, the contextelements may be sequentially parsed or the parsing order may be changed.

Contrarily, in the case of the residual value whose transform_skip_flagis 0, that is, to which the transform has been applied, the contextelements of sig_coeff_flag, par_level_flag, rem_abs_gt1_flag,rem_abs_gt2_flag, abs_remainder, and coeff_sign_flag may be encoded, andthe context elements of sig_coeff_flag, par_level_flag,rem_abs_gt1_flag, rem_abs_gt2_flag, abs_remainder, and coeff_sign_flagmay be parsed (S1220). In this case, the context elements may besequentially parsed or the parsing order may be changed.

That is, in the case of the residual value to which the transform hasnot been applied, par_level_flag, rem_abs_gt1_flag and rem_abs_gt2_flagare not encoded and decoded, when compared with the residual value towhich the transform has been applied. In a case where the residual valueis large, when all of the syntax elements, such as sig_coeff_flag,par_level_flag, rem_abs_gt1_flag and _rem_abs_gt2_flag, are expressed inall coefficients, redundant information is more likely to be transmittedcompared to transmitting the level value by binarizing it as it is, andthus, in the present embodiment, encoding efficiency is improved byomitting the context elements of par_level_flag, rem_abs_gt1_flag andrem_abs_gt2_flag.

Meanwhile, based on the technique of decoding the residual based onTable 1 and the transform skip flag, still another embodiment accordingto the present disclosure proposes a method of determining a contextelement when the current decoding target block is a residual of anuntransformed pixel domain.

In the case of a general transform domain residual, the residual signalis expressed as a level value for each frequency component, and in thehigh frequency region, the probability of being expressed as zero or anumber close to zero by quantization increases. Therefore, in Table 1, amethod was used in which the subsequent context element parsing may beomitted by first encoding sig_coeff_flag, which is a context element forwhether or not the current transform coefficient value is 0.

When sig_coeff_flag is not 0, rem_abs_gt1_flag, par_level_flag,rem_abs_gt2_flag, and the like may be sequentially encoded according tothe value of the current transform coefficient. However, in the case ofa residual signal of a pixel domain that has not been subjected to thetransform, the absolute level value of the signal has randomness.

The context-encoded syntax element may include at least one of thesig_coeff_flag, par_level_flag, rem_abs_gt1_flag, and/orrem_abs_gt2_flag as a syntax element that is encoded through arithmeticcoding based on context. In addition, hereinafter, the context encodingbin may indicate a context-encoded bin for at least one of saidsig_coeff_flag, par_level_flag, rem_abs_gt1_flag, and/orrem_abs_gt2_flag.

In general, in a case where the value of the residual signal is large,when all of the syntax elements, such as sig_coeff_flag, par_level_flag,rem_abs_gt1_flag, and _rem_abs_gt2_flag, are expressed in allcoefficients, redundant information is more likely to be transmittedcompared to transmitting the level value by binarizing it as it is.Accordingly, the present embodiment proposes a method of improvingencoding efficiency by omitting some context elements for the residualsignal of the pixel domain.

The proposed method may be branched based on the transform_skip_flagcontext element in Table 1, and, the existing method that does notcorrespond to the branch statement may follow, for example, the contextelement (sig_coeff_flag, par_level_flag, rem_abs_gt1_flag,rem_abs_gt2_flag, abs_remainder, coeff_sign_flag) of Table 1, or may beencoded and decoded including the context element defined above. Thatis, when the transform is applied, context elements of sig_coeff_flag,par_level_flag, rem_abs_gt1_flag, rem_abs_gt2_flag, abs_remainder, andcoeff_sign_flag may be encoded and decoded as shown in Table 1.

Meanwhile, the residual signal to which the transforma has not beenapplied may be encoded and decoded through context elements ofabs_remainder and coeff_sign_flag.

In addition, according to an example, when all the number of bin for thecontext encoding syntax element has been used, only abs_remainder andcoeff_sign_flag may be coded or decoded/parsed without coding ordecoding/parsing the context encoding syntax element any more.

Table 8 shows the context elements according to the present embodiment.

TABLE 8 Descriptor residual_coding( x0, y0, log2TbWidth, log2TbHeight,cIdx ) {  if( transform_skip_enabled_flag && ( cIdx ! = 0 ||cu_mts_flag[ x0 ][ y0 ] = = 0 ) &&   ( log2TbWidth <= wN ) && (log2TbHeight <= hN ) )   transform_skip_flag[ x0 ][ y0 ][ cIdx ] ae(v)...  if (transform_skip_flag[x0][y0][cIdx]  {   for( n = ( i = =lastSubBlock ) ? lastScanPos − 1 : numSbCoeff − 1; n >= 0; n− − ) {   xC = ( xS << log2SbSize ) + DiagScanOrder[ log2SbSize ][ log2SbSize][ n ][ 0 ]    yC = ( yS << log2SbSize ) + DiagScanOrder[ log2SbSize ][log2SbSize ][ n ][ 1 ]     abs_remainder[ n ]    AbsLevel[ xC ][ yC ] =abs_remainder[ n ]   }   if( !sign_data_hiding_enabled_flag )   signHidden = 0   else    signHidden = ( lastSigScanPosSb −firstSigScanPosSb > 3 ? 1 : 0 )   for( n = numSbCoeff − 1; n >= 0; n− −) {    xC = ( xS << log2SbSize ) + DiagScanOrder[ log2SbSize ][log2SbSize ][ n ][ 0 ]    yC = ( yS << log2SbSize ) + DiagScanOrder[log2SbSize ][ log2SbSize ][ n ][ 1 ]    if( sig_coeff_flag[ xC ][ yC ]&&     ( !signHidden || ( n != firstSigScanPosSb ) ) )    coeff_sign_flag[ n ] ae(v)   }    sumAbsLevel = 0    for( n =numSbCoeff − 1; n >= 0; n− − ) {     xC = ( xS << log2SbSize ) +      DiagScanOrder[ log2SbSize ][ log2SbSize ][ n ][ 0 ]     yC = ( yS<< log2SbSize ) +       DiagScanOrder[ log2SbSize ][ log2SbSize ][ n ][1 ]     TransCoeffLevel[ x0 ][ y0 ][ cIdx ][ xC ][ yC ] =       AbsLevel[ xC ][ yC ] * ( 1 − 2 * coeff_sign_flag[ n ] )     if(signHidden ) {      sumAbsLevel += AbsLevel[ xC ][ yC ]      if( ( n = =firstSigScanPosSb ) && ( sumAbsLevel % 2 ) = = 1 ) )      TransCoeffLevel[ x0 ][ y0 ][ cIdx ][ xC ][ yC ] =         −TransCoeffLevel[ x0 ][ y0 ][ cIdx ][ xC ][ yC ]     }    }   } }  else {  ...  }  if( cu_mts_flag[ x0 ][ y0 ] && ( cIdx = = 0 ) &&  !transform_skip_flag[ x0 ][ y0 ][ cIdx ] &&   ( ( CuPredMode[ x0 ][ y0] = = MODE_INTRA && numSigCoeff > 2 ) ||    ( CuPredMode[ x0 ][ y0 ] = =MODE_INTER ) ) ) {   mts_idx[ x0 ][ y0 ] ae(v) }

FIG. 13 is a control flowchart illustrating a process of parsing acontext element according to the present embodiment. The context elementparsing according to the transform skip flag according to FIG. 13 willbe described as follows.

First, a transform skip flag (transform_skip_flag) indicating whether atransform process has been performed on a transform block is parsed todetermine whether or not transform_skip_flag is 1 (S1300).

As a result of the determination, in the case of the residual valuewhose transform_skip_flag is 1, that is, for which the transform hasbeen skipped without being applied, the context elements ofabs_remainder and coeff_sign_flag may be encoded, and the contextelements of abs_remainder and coeff_sign_flag may be parsed (S1310). Inthis case, the context elements may be sequentially parsed or theparsing order may be changed.

Contrarily, in the case of the residual value whose transform_skip_flagis 0, that is, to which the transform has been applied, the contextelements of sig_coeff_flag, par_level_flag, rem_abs_gt1_flag,rem_abs_gt2_flag, abs_remainder, and coeff_sign_flag may be encoded, andthe context elements of sig_coeff_flag, par_level_flag,rem_abs_gt1_flag, rem_abs_gt2_flag, abs_remainder, and coeff_sign_flagmay be parsed (S1320). In this case, the context elements may besequentially parsed or the parsing order may be changed.

That is, in the case of the residual value to which the transform hasnot been applied, sig_coeff_flag, par_level_flag, rem_abs_gt1_flag andrem_abs_gt2_flag are not encoded and decoded, when compared with theresidual value to which the transform has been applied. In a case wherethe residual value is large, when all of the syntax elements, such assig_coeff_flag, par_level_flag, rem_abs_gt1_flag and _rem_abs_gt2_flag,are expressed in all coefficients, redundant information is more likelyto be transmitted compared to transmitting the level value by binarizingit as it is, and thus, in the present embodiment, encoding efficiency isimproved by omitting the context elements of sig_coeff_flag,par_level_flag, rem_abs_gt1_flag and rem_abs_gt2_flag.

Meanwhile, based on the technique of decoding the residual based onTable 1 and the transform skip flag, still another embodiment accordingto the present disclosure proposes a method of determining a contextelement when the current decoding target block is a residual of anuntransformed pixel domain.

In the case of a general transform domain residual, the residual signalis expressed as a level value for each frequency component, and in thehigh frequency region, the probability of being expressed as zero or anumber close to zero by quantization increases. Therefore, in Table 1, amethod was used in which the subsequent context element parsing may beomitted by first encoding sig_coeff_flag, which is a context element forwhether or not the current transform coefficient value is 0.

When sig_coeff_flag is not 0, rem_abs_gt1_flag, par_level_flag,rem_abs_gt2_flag, and the like may be sequentially encoded according tothe value of the current transform coefficient. However, in the case ofa residual signal of a pixel domain that has not been subjected to thetransform, the absolute level value of the signal has randomness.

The context-encoded syntax element may include at least one of thesig_coeff_flag, par_level_flag, rem_abs_gt1_flag, and/orrem_abs_gt2_flag as a syntax element that is encoded through arithmeticcoding based on context. In addition, hereinafter, the context encodingbin may indicate a context-encoded bin for at least one of saidsig_coeff_flag, par_level_flag, rem_abs_gt1_flag, and/orrem_abs_gt2_flag.

In general, in a case where the value of the residual signal is large,when all of the syntax elements, such as sig_coeff_flag, par_level_flag,rem_abs_gt1_flag, and _rem_abs_gt2_flag, are expressed in allcoefficients, redundant information is more likely to be transmittedcompared to transmitting the level value by binarizing it as it is.Accordingly, the present embodiment proposes a method of improvingencoding efficiency by omitting some context elements for the residualsignal of the pixel domain.

The proposed method may be branched based on the transform_skip_flagcontext element in Table 1, and, the existing method that does notcorrespond to the branch statement may follow, for example, the contextelement (sig_coeff_flag, par_level_flag, rem_abs_gt1_flag,rem_abs_gt2_flag, abs_remainder, coeff_sign_flag) of Table 1, or may beencoded and decoded including the context element defined above. Thatis, when the transform is applied, context elements of sig_coeff_flag,par_level_flag, rem_abs_gt1_flag, rem_abs_gt2_flag, abs_remainder, andcoeff_sign_flag may be encoded and decoded as shown in Table 1.

Meanwhile, the residual signal to which the transforma has not beenapplied may be encoded and decoded through context elements ofsig_coeff_flag, rem_abs_gt1_flag, abs_remainder, and coeff_sign_flag.

Table 9 shows the context elements according to the present embodiment.

TABLE 9 Descriptor residual_coding( x0, y0, log2TbWidth, log2TbHeight,cIdx ) {  if( transform_skip_enabled_flag && ( cIdx ! = 0 ||cu_mts_flag[ x0 ][ y0 ] = = 0 ) &&   ( log2TbWidth <= wN ) && (log2TbHeight <= hN ) )   transform_skip_flag[ x0 ][ y0 ][ cIdx ] ae(v)... if ( transform_skip_flag[x0][y0][cIdx] ) {   for( n = ( i = =lastSubBlock ) ? lastScanPos − 1 : numSbCoeff − 1; n >= 0; n− − ) {   xC = ( xS << log2SbSize ) + DiagScanOrder[ log2SbSize ][ log2SbSize][ n ][ 0 ]    yC = ( yS << log2SbSize ) + DiagScanOrder[ log2SbSize ][log2SbSize ][ n ][ 1 ]    if( coded_sub_block_flag[ xS ][ yS ] && ( n >0 || !inferSbDcSigCoeffFlag ) ) {     sig_coeff_flag[ xC ][ yC ] ae(v)   }    if( sig_coeff_flag[ xC ][ yC ] ) {     rem_abs_gt1_flag[ n ]ae(v)     if( lastSigScanPosSb = = −1 )      lastSigScanPosSb = n    firstSigScanPosSb = n    }    AbsLevelPass1[ xC ][ yC ] =     sig_coeff_flag[ xC][ yC ] + rem_abs_gt1_flag[ n ]   }   for( n =numSbCoeff − 1; n >= 0; n− − ) {    xC = ( xS << log2SbSize ) +DiagScanOrder[ log2SbSize ][ log2SbSize ][ n ][ 0 ]    yC = ( yS <<log2SbSize ) + DiagScanOrder[ log2SbSize ][ log2SbSize ][ n ][ 1 ]   if( rem_abs_gt1_flag[ n ] )     abs_remainder[ n ] ae(v)    AbsLevel[xC ][ yC ] = AbsLevelPass1[ xC ][ yC ] +           abs_remainder[ n ]  }   signHidden = ( lastSigScanPosSb − firstSigScanPosSb > 3 ? 1 : 0 )  for( n = numSbCoeff − 1; n >= 0; n− − ) {    xC = ( xS << log2SbSize) + DiagScanOrder[ log2SbSize ][ log2SbSize ][ n ][ 0 ]    yC = ( yS <<log2SbSize ) + DiagScanOrder[ log2SbSize ][ log2SbSize ][ n ][ 1 ]   if( sig_coeff_flag[ xC ][ yC ] &&     ( !signHidden || ( n !=firstSigScanPosSb ) ) )     coeff_sign_flag[ n ] ae(v)   }   sumAbsLevel = 0    for( n = numSbCoeff − 1; n >= 0; n− − ) {     xC =( xS << log2SbSize ) +       DiagScanOrder[ log2SbSize ][ log2SbSize ][n ][ 0 ]     yC = ( yS << log2SbSize ) +       DiagScanOrder[ log2SbSize][ log2SbSize ][ n ][ 1 ]     if( sig_coeff_flag[ xC ][ yC ] ) {     TransCoeffLevel[ x0 ][ y0 ][ cIdx ][ xC ][ yC ] =        AbsLevel[xC ][ yC ] * ( 1 − 2 * coeff_sign_flag[ n ] )      if( signHidden ) {      sumAbsLevel += AbsLevel[ xC][ yC ]       if( ( n = =firstSigScanPosSb ) && ( sumAbsLevel % 2 ) = = 1 ) )       TransCoeffLevel[ x0 ][ y0 ][ cIdx ][ xC ][ yC ] =         −TransCoeffLevel[ x0 ][ y0 ][ cIdx ][ xC ][ yC ]      }     }   }  } } else { //!transform_skip_flag ... }  if( cu_mts_flag[ x0 ][ y0] && ( cIdx = = 0 ) &&   !transform_skip_flag[ x0 ][ y0 ][ cIdx ] &&   (( CuPredMode[ x0 ][ y0 ] = = MODE_INTRA && numSigCoeff > 2 ) ||    (CuPredMode[ x0 ][ y0 ] = = MODE_INTER ) ) ) {   mts_idx[ x0 ][ y0 ]ae(v) }

FIG. 14 is a control flowchart illustrating a process of parsing acontext element according to the present embodiment. The context elementparsing according to the transform skip flag according to FIG. 14 willbe described as follows.

First, a transform skip flag (transform_skip_flag) indicating whether atransform process has been performed on a transform block is parsed todetermine whether or not transform_skip_flag is 1 (S1400).

As a result of the determination, in the case of the residual valuewhose transform_skip_flag is 1, that is, for which the transform hasbeen skipped without being applied, the context elements ofsig_coeff_flag, rem_abs_gt1_flag, abs_remainder, and coeff_sign_flag maybe encoded, and the context elements of sig_coeff_flag,rem_abs_gt1_flag, abs_remainder, and coeff_sign_flag may be parsed(S1410). In this case, the context elements may be sequentially parsedor the parsing order may be changed.

Contrarily, in the case of the residual value whose transform_skip_flagis 0, that is, to which the transform has been applied, the contextelements of sig_coeff_flag, par_level_flag, rem_abs_gt1_flag,rem_abs_gt2_flag, abs_remainder, and coeff_sign_flag may be encoded, andthe context elements of sig_coeff_flag, par_level_flag,rem_abs_gt1_flag, rem_abs_gt2_flag, abs_remainder, and coeff_sign_flagmay be parsed (S1420). In this case, the context elements may besequentially parsed or the parsing order may be changed.

That is, in the case of the residual value to which the transform hasnot been applied, par_level_flag and rem_abs_gt2_flag are not encodedand decoded, when compared with the residual value to which thetransform has been applied. In a case where the residual value is large,when all of the syntax elements, such as sig_coeff_flag, par_level_flag,rem_abs_gt1_flag and rem_abs_gt2_flag, are expressed in allcoefficients, redundant information is more likely to be transmittedcompared to transmitting the level value by binarizing it as it is, andthus, in the present embodiment, encoding efficiency is improved byomitting the context elements of par_level_flag and rem_abs_gt2_flag.

Meanwhile, based on the technique of decoding the residual based onTable 1 and the transform skip flag, still another embodiment accordingto the present disclosure proposes a method of determining a contextelement when the current decoding target block is a residual of anuntransformed pixel domain.

In the case of a general transform domain residual, the residual signalis expressed as a level value for each frequency component, and in thehigh frequency region, the probability of being expressed as zero or anumber close to zero by quantization increases. Therefore, in Table 1, amethod was used in which the subsequent context element parsing may beomitted by first encoding sig_coeff_flag, which is a context element forwhether or not the current transform coefficient value is 0.

When sig_coeff_flag is not 0, rem_abs_gt1_flag, par_level_flag,rem_abs_gt2_flag, and the like may be sequentially encoded according tothe value of the current transform coefficient. However, in the case ofa residual signal of a pixel domain that has not been subjected to thetransform, the absolute level value of the signal has randomness.

The context-encoded syntax element may include at least one of thesig_coeff_flag, par_level_flag, rem_abs_gt1_flag, and/orrem_abs_gt2_flag as a syntax element that is encoded through arithmeticcoding based on context. In addition, hereinafter, the context encodingbin may indicate a context-encoded bin for at least one of saidsig_coeff_flag, par_level_flag, rem_abs_gt1_flag, and/orrem_abs_gt2_flag.

In general, in a case where the value of the residual signal is large,when all of the syntax elements, such as sig_coeff_flag, par_level_flag,rem_abs_gt1_flag, and rem_abs_gt2_flag, are expressed in allcoefficients, redundant information is more likely to be transmittedcompared to transmitting the level value by binarizing it as it is.Accordingly, the present embodiment proposes a method of improvingencoding efficiency by omitting some context elements for the residualsignal of the pixel domain.

The proposed method may be branched based on the transform_skip_flagcontext element in Table 1, and, the existing method that does notcorrespond to the branch statement may follow, for example, the contextelement (sig_coeff_flag, par_level_flag, rem_abs_gt1_flag,rem_abs_gt2_flag, abs_remainder, coeff_sign_flag) of Table 1, or may beencoded and decoded including the context element defined above. Thatis, when the transform is applied, context elements of sig_coeff_flag,par_level_flag, rem_abs_gt1_flag, rem_abs_gt2_flag, abs_remainder, andcoeff_sign_flag may be encoded and decoded as shown in Table 1.

Meanwhile, the residual signal to which the transforma is not appliedmay be encoded and decoded through context elements of sig_coeff_flag,rem_abs_gt1_flag, rem_abs_gt2_flag, abs_remainder, and coeff_sign_flag.

Table 10 shows the context elements according to the present embodiment.

TABLE 10 Descriptor residual_coding( x0, y0, log2TbWidth, log2TbHeight,cIdx ) {  if( transform_skip_enabled_flag && ( cIdx ! = 0 ||cu_mts_flag[ x0 ][ y0 ] = = 0 ) &&   ( log2TbWidth <= wN ) && (log2TbHeight <= hN ) )   transform_skip_flag[ x0 ][ y0 ][ cIdx ] ae(v)... if ( transform_skip_flag[x0][y0][cIdx] ) {   for( n = ( i = =lastSubBlock ) ? lastScanPos − 1 : numSbCoeff − 1; n >= 0; n− − ) {   xC = ( xS << log2SbSize ) + DiagScanOrder[ log2SbSize ][ log2SbSize][ n ][ 0 ]    yC = ( yS << log2SbSize ) + DiagScanOrder[ log2SbSize ][log2SbSize ][ n ][ 1 ]    if( coded_sub_block_flag[ xS ][ yS ] && ( n >0 || !inferSbDcSigCoeffFlag ) ) {     sig_coeff_flag[ xC ][ yC ] ae(v)   }    if( sig_coeff_flag[ xC ][ yC ] ) {     rem_abs_gt1_flag[ n ]ae(v)     if( lastSigScanPosSb = = −1 )      lastSigScanPosSb = n    firstSigScanPosSb = n    }    AbsLevelPass1[ xC ][ yC ] =     sig_coeff_flag[ xC][ yC ] + rem_abs_gt1_flag[ n ]    }    for( n =numSbCoeff − 1; n >= 0; n− − ) {     if( rem_abs_gt1_flag[ n ] )     rem_abs_gt2_flag[ n ] ae(v)    }    for( n = numSbCoeff − 1; n >=0; n− − ) {     xC = ( xS << log2SbSize ) +  DiagScanOrder[ log2SbSize][ log2SbSize ][ n ][ 0 ]     yC = ( yS << log2SbSize ) + DiagScanOrder[ log2SbSize ][ log2SbSize ][ n ][ 1 ]     if(rem_abs_gt2_flag[ n ] )      abs_remainder[ n ] ae(v)     AbsLevel[ xC][ yC ] = AbsLevelPass1[ xC ][ yC ] + rem_abs_gt2_flag[ n ]          abs_remainder[ n ]    }    signHidden = ( lastSigScanPosSb −firstSigScanPosSb > 3 ? 1 : 0 )    for( n = numSbCoeff − 1; n >= 0; n− −) {     xC = ( xS << log2SbSize ) +  DiagScanOrder[ log2SbSize ][log2SbSize ][ n ][ 0 ]     yC = ( yS << log2SbSize ) +  DiagScanOrder[log2SbSize ][ log2SbSize ][ n ][ 1 ]     if( sig_coeff_flag[ xC ][ yC ]&&      ( !signHidden || ( n != firstSigScanPosSb ) ) )     coeff_sign_flag[ n ] ae(v)    }    sumAbsLevel = 0    for( n =numSbCoeff − 1; n >= 0; n− − ) {     xC = ( xS << log2SbSize ) +      DiagScanOrder[ log2SbSize ][ log2SbSize ][ n ][ 0 ]     yC = ( yS<< log2SbSize ) +       DiagScanOrder[ log2SbSize ][ log2SbSize ][ n ][1 ]     if( sig_coeff_flag[ xC ][ yC ] ) {      TransCoeffLevel[ x0 ][y0 ][ cIdx ][ xC ][ yC ] =        AbsLevel[ xC ][ yC ] * ( 1 − 2 *coeff_sign_flag[ n ] )      if( signHidden ) {       sumAbsLevel +=AbsLevel[ xC][ yC ]       if( ( n = = firstSigScanPosSb ) && (sumAbsLevel % 2 ) = = 1 ) )        TransCoeffLevel[ x0 ][ y0 ][ cIdx ][xC ][ yC ] =          −TransCoeffLevel[ x0 ][ y0 ][ cIdx ][ xC ][ yC ]     }     }    }  } } else { //!transform_skip_flag ... }  if(cu_mts_flag[ x0 ][ y0 ] && ( cIdx = = 0 ) &&   !transform_skip_flag[ x0][ y0 ][ cIdx ] &&   ( ( CuPredMode[ x0 ][ y0 ] = = MODE_INTRA &&numSigCoeff > 2 ) ||    ( CuPredMode[ x0 ][ y0 ] = = MODE_INTER ) ) ) {  mts_idx[ x0 ][ y0 ] ae(v) }

FIG. 15 is a control flowchart illustrating a process of parsing acontext element according to the present embodiment. The context elementparsing according to the transform skip flag according to FIG. 15 willbe described as follows.

First, a transform skip flag (transform_skip_flag) indicating whether atransform process has been performed on a transform block is parsed todetermine whether or not transform_skip_flag is 1 (S1500).

As a result of the determination, in the case of the residual valuewhose transform_skip_flag is 1, that is, for which the transform hasbeen skipped without being applied, the context elements ofsig_coeff_flag, rem_abs_gt1_flag, rem_abs_gt2_flag, abs_remainder, andcoeff_sign_flag may be encoded, and the context elements ofsig_coeff_flag, rem_abs_gt1_flag, rem_abs_gt2_flag, abs_remainder, andcoeff_sign_flag may be parsed (S1510). In this case, the contextelements may be sequentially parsed or the parsing order may be changed.

Contrarily, in the case of the residual value whose transform_skip_flagis 0, that is, to which the transform has been applied, the contextelements of sig_coeff_flag, par_level_flag, rem_abs_gt1_flag,rem_abs_gt2_flag, abs_remainder, and coeff_sign_flag may be encoded, andthe context elements of sig_coeff_flag, par_level_flag,rem_abs_gt1_flag, rem_abs_gt2_flag, abs_remainder, and coeff_sign_flagmay be parsed (S1520). In this case, the context elements may besequentially parsed or the parsing order may be changed.

That is, in the case of the residual value to which the transform is notapplied, par_level_flag is not encoded and decoded, when compared withthe residual value to which the transform is applied. In a case wherethe residual value is large, when all of the syntax elements, such assig_coeff_flag, par_level_flag, rem_abs_gt1_flag and rem_abs_gt2_flag,are expressed in all coefficients, redundant information is more likelyto be transmitted compared to transmitting the level value by binarizingit as it is, and thus, in the present embodiment, encoding efficiency isimproved by omitting the context element of par_level_flag.

Syntax elements rem_abs_gt1_flag and rem_abs_gt2_flag may be representedbased on abs_level_gtx_flag[n][j] as described above, and may also beexpressed as abs_rem_gt1_flag and abs_rem_gt2_flag, or abs_rem_gtx_flag.

As described above, according to embodiments of the present disclosure,different residual coding schemes, that is, residual syntax, may beapplied depending on whether or not transform skip is applied forresidual coding.

For example, the signaling order of the flag (coeff_sign_flag) for thesign of the transform coefficient may be different depending on whetheror not the transform skip is applied. When transformation skip is notapplied, coeff_sign_flag is signaled after abs_remainder, while whentransform skip is applied, coeff_sign_flag may be signaled beforerem_abs_gt1_flag.

In addition, for example, rem_abs_gt1_flag, rem_abs_gt2_flag, that is,rem_abs_gtx_flag parsing and a parsing loop for abs_remainder may varydepending on whether transform skip is applied.

Additionally, the context syntax element encoded through arithmeticcoding based on context may include a significant coefficient flag(sig_coeff_flag) indicating whether or not the quantized transformcoefficient is a non-zero significant coefficient, a parity level flag(par_level_flag) for parity of a transform coefficient level for thequantized transform coefficient, a first transform coefficient levelflag (rem_abs_gt1_flag) for whether or not the transform coefficientlevel is greater than a first threshold, and a second transformcoefficient level flag (rem_abs_gt2_flag) for whether the transformcoefficient level of the quantized transform coefficient is greater thana second threshold. In this case, the decoding of the first transformcoefficient level flag may be performed prior to the decoding of theparity level flag.

Tables 11 to 13 show the context elements according to above-describedexample.

TABLE 11    transform_skip_flag[ x0 ][ y0 ] ae(v)   if( ( ( CuPredMode[chType ][ x0 ][ y0 ] = = MODE_INTER &&   sps_explicit_mts_inter_enabled_flag )    || ( CuPredMode[ chType ][x0 ][ y0 ] = = MODE_INTRA &&    sps_explicit_mts_intra_enabled_flag ) )&& ( !transform_skip_flag[ x0 ][ y0 ] ) )    tu_mts_idx[ x0 ][ y0 ]ae(v)  }  if( tu_cbf_luma[ x0 ][ y0 ] ) {   if( !transform_skip_flag[ x0][ y0 ] )    residual_coding( x0, y0, Log2( tbWidth ), Log2( tbHeight ),0 )   else    residual_ts_coding( x0, y0, Log2( tbWidth ), Log2(tbHeight ), 0 )  }  if( tu_cbf_cb[ x0 ][ y0 ] )   residual_coding( xC,yC, Log2( wC ), Log2( hC ), 1 )  if( tu_cbf_cr[ x0 ][ y0 ] &&   !(tu_cbf_cb[ x0 ][ y0 ] && tu_joint_cbcr_residual_flag[ x0 ][ y0 ] )) {  residual_coding( xC, yC, Log2( wC ), Log2( hC ), 2 )  } }

TABLE 12 Descriptor residual_coding( x0, y0, log2TbWidth, log2TbHeight,cIdx ) {  if( ( tu_mts_idx[ x0 ][ y0 ] > 0 ||    ( cu_sbt_flag &&log2TbWidth < 6 && log2TbHeight < 6 ) )     && cIdx = = 0 &&log2TbWidth > 4 )   log2ZoTbWidth = 4  else   log2ZoTbWidth = Min(log2TbWidth, 5 )  MaxCcbs = 2 * ( 1 << log2TbWidth ) * ( 1<<log2TbHeight )  if( tu_mts_idx[ x0 ][ y0 ] > 0 ||    ( cu_sbt_flag &&log2TbWidth < 6 && log2TbHeight < 6 ) )     && cIdx = = 0 &&log2TbHeight > 4 )   log2ZoTbHeight = 4  else   log2ZoTbHeight = Min(log2TbHeight, 5 )  if( log2TbWidth > 0 )   last_sig_coeff_x_prefix ae(v) if( log2TbHeight > 0 )   last_sig_coeff_y_prefix ae(v)  if(last_sig_coeff_x_prefix > 3 )   last_sig_coeff_x_suffix ae(v)  if(last_sig_coeff_y_prefix > 3 )   last_sig_coeff_y_suffix ae(v) log2TbWidth = log2ZoTbWidth  log2TbHeight = log2ZoTbHeight remBinsPass1 = ( ( 1 << ( log2TbWidth + log2TbHeight ) ) * 7 ) >> 2 log2SbW = ( Min( log2TbWidth, log2TbHeight ) < 2 ? 1 : 2 )  log2SbH =log2SbW  if( log2TbWidth + log2TbHeight > 3 ) {   if( log2TbWidth < 2 ){    log2SbW = log2TbWidth    log2SbH = 4 − log2SbW   } else if(log2TbHeight < 2 ) {    log2SbH = log2TbHeight    log2SbW = 4 − log2SbH  }  }  numSbCoeff = 1 << ( log2SbW + log2SbH )  lastScanPos =numSbCoeff  lastSubBlock = ( 1 << ( log2TbWidth + log2TbHeight − (log2SbW + log2SbH ) ) ) − 1  do {   if( lastScanPos = = 0 ) {   lastScanPos = numSbCoeff    lastSubBlock− −   }   lastScanPos− −   xS= DiagScanOrder[ log2TbWidth − log2SbW ][ log2TbHeight − log2SbH ]        [ lastSubBlock ][ 0 ]   yS = DiagScanOrder[ log2TbWidth −log2SbW ][ log2TbHeight − log2SbH ]         [ lastSubBlock ][ 1 ]   xC =( xS << log2SbW ) + DiagScanOrder[ log2SbW ][ log2SbH ][ lastScanPos ][0 ]   yC = ( yS << log2SbH ) + DiagScanOrder[ log2SbW ][ log2SbH ][lastScanPos ][ 1 ]  } while( ( xC != LastSignificantCoeffX ) || ( yC !=LastSignificantCoeffY ) )  if( lastSubBlock = = 0 && log2TbWidth >= 2 &&log2TbHeight >= 2 &&   !transform_skip_flag[ x0 ][ y0 ] && lastScanPos >0 )   LfnstDcOnly = 0  if( ( lastSubBlock > 0 && log2TbWidth >= 2 &&log2TbHeight >= 2 ) ||   ( lastScanPos > 7 && ( log2TbWidth = = 2 ||log2TbWidth = = 3 ) &&   log2TbWidth = = log2TbHeight ) )  LfnstZeroOutSigCoeffFlag = 0  QState = 0  for( i = lastSubBlock; i >=0; i− − ) {   startQStateSb = QState   xS = DiagScanOrder[ log2TbWidth −log2SbW ][ log2TbHeight − log2SbH ]         [ i ][ 0 ]   yS =DiagScanOrder[ log2TbWidth − log2SbW ][ log2TbHeight − log2SbH ]        [ i ][ 1 ]   inferSbDcSigCoeffFlag = 0   if( ( i < lastSubBlock) && ( i > 0 ) ) {    coded_sub_block_flag[ xS ][ yS ] ae(v)   inferSbDcSigCoeffFlag = 1   }   firstSigScanPosSb = numSbCoeff  lastSigScanPosSb = −1   firstPosMode0 = ( i = = lastSubBlock ?lastScanPos : numSbCoeff − 1 )   firstPosMode1 = −1   for( n =firstPosMode0; n >= 0 && remBinsPass1 >= 4; n− − ) {    xC = ( xS <<log2SbW ) + DiagScanOrder[ log2SbW ][ log2SbH ][ n ][ 0 ]    yC = ( yS<< log2SbH ) + DiagScanOrder[ log2SbW ][ log2SbH ][ n ][ 1 ]    if(coded_sub_block_flag[ xS ][ yS ] && ( n > 0 || !inferSbDcSigCoeffFlag )&&     ( xC != LastSignificantCoeffX || yC != LastSignificantCoeffY ) ){     sig_coeff_flag[ xC ][ yC ] ae(v)     remBinsPass1− −     if(sig_coeff_flag[ xC ][ yC ] )      inferSbDcSigCoeffFlag = 0    }    if(sig_coeff_flag[ xC ][ yC ] ) {     abs_level_gtx_flag[ n ][ 0 ] ae(v)    remBinsPass1− −     if( abs_level_gtx_flag[ n ][ 0 ] ) {     par_level_flag[ n ] ae(v)      remBinsPass1− −     abs_level_gtx_flag[ n ][ 1 ] ae(v)      remBinsPass1− −     }    if( lastSigScanPosSb = = −1 )      lastSigScanPosSb = n    firstSigScanPosSb = n    }    AbsLevelPass1[ xC ][ yC ] =sig_coeff_flag[ xC][ yC ] + par_level_flag[ n ] +          abs_level_gtx_flag[ n ][ 0 ] + 2 * abs_level_gtx_flag[ n ][ 1]    if( dep_quant_enabled_flag )     QState = QStateTransTable[ QState][ AbsLevelPass1[ xC ][ yC ] & 1 ]    if( remBinsPass1 < 4 )    firstPosMode1 = n − 1   }   for( n = numSbCoeff − 1; n >=firstPosMode1; n− − ) {    xC = ( xS << log2SbW ) + DiagScanOrder[log2SbW ][ log2SbH ][ n ][ 0 ]    yC = ( yS << log2SbH ) +DiagScanOrder[ log2SbW ][ log2SbH ][ n ][ 1 ]    if( abs_level_gtx_flag[n ][ 1 ] )     abs_remainder[ n ] ae(v)    AbsLevel[ xC ][ yC ] =AbsLevelPass1[ xC ][ yC ] +2 * abs_remainder[ n ]   }   for( n =firstPosMode1; n >= 0; n− − ) {    xC = ( xS << log2SbW ) +DiagScanOrder[ log2SbW ][ log2SbH ][ n ][ 0 ]    yC = ( yS << log2SbH) + DiagScanOrder[ log2SbW ][ log2SbH ][ n ][ 1 ]    dec_abs_level[ n ]ae(v)    if(AbsLevel[ xC ][ yC ] > 0 )     firstSigScanPosSb = n    if(dep_quant_enabled_flag )     QState = QStateTransTable[ QState ][AbsLevel[ xC ][ yC ] & 1 ]   }   if( dep_quant_enabled_flag ||!sign_data_hiding_enabled_flag )    signHidden = 0   else    signHidden= ( lastSigScanPosSb − firstSigScanPosSb > 3 ? 1 : 0 )   for( n =numSbCoeff − 1; n >= 0; n− − ) {    xC = ( xS << log2SbW ) +DiagScanOrder[ log2SbW ][ log2SbH ][ n ][ 0 ]    yC = ( yS << log2SbH) + DiagScanOrder[ log2SbW ][ log2SbH ][ n ][ 1 ]    if( (AbsLevel[ xC][ yC ] > 0 ) &&     ( !signHidden || ( n != firstSigScanPosSb ) ) )    coeff_sign_flag[ n ] ae(v)   }   if( dep_quant_enabled_flag ) {   QState = startQStateSb    for( n = numSbCoeff − 1; n >= 0; n− − ) {    xC = ( xS << log2SbW ) + DiagScanOrder[ log2SbW ][ log2SbH ][ n ][ 0]     yC = ( yS << log2SbH ) + DiagScanOrder[ log2SbW ][ log2SbH ][ n ][1 ]     if( AbsLevel[ xC ][ yC ] > 0 )      TransCoeffLevel[ x0 ][ y0 ][cIdx ][ xC ][ yC ] =        ( 2 * AbsLevel[ xC ][ yC ] − ( QState > 1 ?1 : 0 ) ) *        ( 1 − 2 * coeff_sign_flag[ n ] )     QState =QStateTransTable[ QState ][ par_level_flag[ n ] ]   } else {   sumAbsLevel = 0    for( n = numSbCoeff − 1; n >= 0; n− − ) {     xC =( xS << log2SbW ) + DiagScanOrder[ log2SbW ][ log2SbH ][ n ][ 0 ]     yC= ( yS << log2SbH ) + DiagScanOrder[ log2SbW ][ log2SbH ][ n ][ 1 ]    if( AbsLevel[ xC ][ yC ] > 0 ) {      TransCoeffLevel[ x0 ][ y0 ][cIdx ][ xC ][ yC ] =        AbsLevel[ xC ][ yC ] * ( 1 − 2 *coeff_sign_flag[ n ] )      if( signHidden ) {       sumAbsLevel +=AbsLevel[ xC][ yC ]       if( ( n = = firstSigScanPosSb ) && (sumAbsLevel % 2 ) = = 1 ) )        TransCoeffLevel[ x0 ][ y0 ][ cIdx ][xC ][ yC ] =           −TransCoeffLevel[ x0 ][ y0 ][ cIdx ][ xC ][ yC ]     }     }    }   }  } }

TABLE 13 Descriptor residual_ts_coding( x0, y0, log2TbWidth,log2TbHeight, cIdx ) {  log2SbSize = ( Min( log2TbWidth, log2TbHeight )< 2 ? 1 : 2 )  numSbCoeff = 1 <<( log2SbSize << 1 )  lastSubBlock = ( 1<< ( log2TbWidth + log2TbHeight − 2 * log2SbSize ) ) − 1  inferSbCbf = 1 MaxCcbs = 2 * ( 1 << log2TbWidth ) * ( 1<< log2TbHeight )  for( i =0; i<= lastSubBlock; i++ ) {   xS = DiagScanOrder[ log2TbWidth − log2SbSize][ log2TbHeight − log2SbSize ][ i ][ 0 ]   yS = DiagScanOrder[log2TbWidth − log2SbSize ][ log2TbHeight − log2SbSize ][ i ][ 1 ]   if(( i != lastSubBlock || !inferSbCbf ) {    coded_sub_block_flag[ xS ][ yS] ae(v)   }   if( coded_sub_block_flag[ xS ][ yS ] && i < lastSubBlock )   inferSbCbf = 0  /* First scan pass */   inferSbSigCoeffFlag = 1  for( n = 0; n <= numSbCoeff − 1; n++ ) {    xC = ( xS << log2SbSize) + DiagScanOrder[ log2SbSize ][ log2SbSize ][ n ][ 0 ]    yC = ( yS <<log2SbSize ) + DiagScanOrder[ log2SbSize ][ log2SbSize ][ n ][ 1 ]   if( coded_sub_block_flag[ xS ][ yS ] &&     ( n != numSbCoeff − 1 ||!inferSbSigCoeffFlag ) ) {     sig_coeff_flag[ xC ][ yC ] ae(v)    MaxCcbs− −     if( sig_coeff_flag[ xC ][ yC ] )     inferSbSigCoeffFlag = 0    }    CoeffSignLevel[ xC ][ yC ] = 0   if( sig_coeff_flag[ xC ][ yC ] {     coeff_sign_flag[ n ] ae(v)    MaxCcbs− −     CoeffSignLevel[ xC ][ yC ] = ( coeff_sign_flag[ n ] >0 ? −1 : 1 )     abs_level_gtx_flag[ n ][ 0 ] ae(v)     MaxCcbs− −    if( abs_level_gtx_flag[ n ][ 0 ] ) {      par_level_flag[ n ] ae(v)     MaxCcbs− −     }    }    AbsLevelPassX[ xC ][ yC ] =     sig_coeff_flag[ xC ][ yC ] + par_level_flag[ n ] +abs_level_gtx_flag[ n ][ 0 ]   }  /* Greater than X scan pass(numGtXFlags=5) */   for( n = 0; n <= numSbCoeff − 1; n++ ) {    xC = (xS << log2SbSize ) + DiagScanOrder[ log2SbSize ][ log2SbSize ][ n ][ 0 ]   yC = ( yS << log2SbSize ) + DiagScanOrder[ log2SbSize ][ log2SbSize][ n ][ 1 ]    for( j = 1; j < 5; j++ ) {     if( abs_level_gtx_flag[ n][ j − 1 ] )      abs_level_gtx_flag[ n ][ j ] ae(v)     MaxCcbs− −    AbsLevelPassX[ xC ][ yC ] + = 2 * abs_level_gtx_flag[ n ][ j ]    }  }  /* remainder scan pass */   for( n = 0; n <= numSbCoeff − 1; n++ ){    xC = ( xS << log2SbSize ) + DiagScanOrder[ log2SbSize ][ log2SbSize][ n ][ 0 ]    yC = ( yS << log2SbSize ) + DiagScanOrder[ log2SbSize ][log2SbSize ][ n ][ 1 ]    if( abs_level_gtx_flag[ n ][ 4 ] )    abs_remainder[ n ] ae(v)    if( intra_bdpcm_flag = = 0 ) {    absRightCoeff = abs( TransCoeffLevel[ x0 ][ y0 ][ cIdx ][ xC − 1 ][yC ] )     absBelowCoeff = abs( TransCoeffLevel[ x0 ][ y0 ][ cIdx ][ xC][ yC − 1 ] )     predCoeff = Max( absRightCoeff, absBelowCoeff )    if( AbsLevelPassX[ xC ][ yC ] + abs_remainder[ n ] = = 1 &&predCoeff > 0 )      TransCoeffLevel[ x0 ][ y0 ][ cIdx ][ xC ][ yC ] =          ( 1 − 2 * coeff_sign_flag[ n ] ) * predCoeff     else if(AbsLevelPassX[ xC ][ yC ] + abs_remainder[ n ] <= predCoeff )     TransCoeffLevel[ x0 ][ y0 ][ cIdx ][ xC ][ yC ] = ( 1 − 2 *coeff_sign_flag[ n ] ) *           ( AbsLevelPassX[ xC ][ yC ] +abs_remainder[ n ] − 1)     else      TransCoeffLevel[ x0 ][ y0 ][ cIdx][ xC ][ yC ] = ( 1 − 2 * coeff_sign_flag[ n ] ) *           (AbsLevelPassX[ xC ][ yC ] + abs_remainder[ n ] )    } else    TransCoeffLevel[ x0 ][ y0 ][ cIdx ][ xC ][ yC ] = ( 1 − 2 *coeff_sign_flag[ n ] ) *           ( AbsLevelPassX[ xC ][ yC ] +abs_remainder[ n ] )   }  } }

Table 11 shows that residual coding is branched according to the valueof transform_skip_flag, that is, different syntax elements are used forthe residual. In addition, Table 12 shows residual coding in the case oftransform_skip_flag having a value of 0, that is, in the case of thetransform being applied, and Table 13 shows residual coding in the caseof transform_skip_flag having a value of 1, that is, in the case of thetransform not being applied.

In Tables 12 and 13, par_level_flag may be expressed as Equation 6below.par_level_flag=coeff & 1  [Equation 6]

In addition, in Tables 12 and 13, since par_level_flag is parsed, thatis, decoded after abs_level_gtx_flag, rem_abs_gt1_flag may indicatewhether or not the transform coefficient at the corresponding scanningposition n is greater than 1, and rem_abs_gt2_flag may indicate whetheror not the transform coefficient at the corresponding scanning positionn is greater than 3. That is, rem_abs_gt2_flag in Table 1 may beexpressed as rem_abs_gt3_flag in Tables 12 and 13.

When Equations 2 to 3 are changed as described above, Equation 4 may bechanged as follows in the case of following Tables 12 and 13.|coeff|=sig_coeff_flag+par_level_flag+rem_abs_gt1_flag+2*(rem_abs_gt2_flag+abs_remainder  [Equation7]

FIG. 16 represents an example of a contents streaming system to whichthe present document may be applied.

Referring to FIG. 16 , the content streaming system to which the presentdocument is applied may generally include an encoding server, astreaming server, a web server, a media storage, a user device, and amultimedia input device.

The encoding server functions to compress to digital data the contentsinput from the multimedia input devices, such as the smart phone, thecamera, the camcorder and the like, to generate a bitstream, and totransmit it to the streaming server. As another example, in a case wherethe multimedia input device, such as, the smart phone, the camera, thecamcorder or the like, directly generates a bitstream, the encodingserver may be omitted.

The bitstream may be generated by an encoding method or a bitstreamgeneration method to which the present document is applied. And thestreaming server may temporarily store the bitstream in a process oftransmitting or receiving the bitstream.

The streaming server transmits multimedia data to the user equipment onthe basis of a user's request through the web server, which functions asan instrument that informs a user of what service there is. When theuser requests a service which the user wants, the web server transfersthe request to the streaming server, and the streaming server transmitsmultimedia data to the user. In this regard, the contents streamingsystem may include a separate control server, and in this case, thecontrol server functions to control commands/responses betweenrespective equipments in the content streaming system.

The streaming server may receive contents from the media storage and/orthe encoding server. For example, in a case the contents are receivedfrom the encoding server, the contents may be received in real time. Inthis case, the streaming server may store the bitstream for apredetermined period of time to provide the streaming service smoothly.

For example, the user equipment may include a mobile phone, a smartphone, a laptop computer, a digital broadcasting terminal, a personaldigital assistant (PDA), a portable multimedia player (PMP), anavigation, a slate PC, a tablet PC, an ultrabook, a wearable device(e.g., a watch-type terminal (smart watch), a glass-type terminal (smartglass), a head mounted display (HMD)), a digital TV, a desktop computer,a digital signage or the like.

Each of servers in the contents streaming system may be operated as adistributed server, and in this case, data received by each server maybe processed in distributed manner.

What is claimed is:
 1. An image decoding method performed by a decodingapparatus, the method comprising: obtaining residual information from abitstream; deriving a quantized transform coefficient for a currentblock based on the residual information; deriving a transformcoefficient for the current block by performing a dequantization for thequantized transform coefficient; deriving a residual sample for thecurrent block based on the transform coefficient; and generating areconstructed picture based on the residual sample for the currentblock, wherein the residual information includes transform coefficientlevel flags related to whether or not a transform coefficient level forthe quantized transform coefficient is greater than predeterminedthresholds, and wherein a number of the transform coefficient levelflags is based on whether or not a transform is applied to the currentblock.
 2. The image decoding method of claim 1, wherein the residualinformation includes a first transform coefficient level flag related towhether or not the transform coefficient level for the quantizedtransform coefficient is greater than a first threshold value, and asecond transform coefficient level flag related to whether or not thetransform coefficient level for the quantized transform coefficient isgreater than a second threshold value, and wherein the second transformcoefficient level flag is decoded in different ways based on whether ornot the transform is applied to the current block.
 3. The image decodingmethod of claim 1, wherein the residual information includes a contextsyntax element coded based on a context, and wherein the context syntaxelement includes a significant coefficient flag related to whether ornot the quantized transform coefficient is a non-zero significantcoefficient, a parity level flag related to a parity of the transformcoefficient level for the quantized transform coefficient, a firsttransform coefficient level flag related to whether or not the transformcoefficient level for the quantized transform coefficient is greaterthan a first threshold value, and a second transform coefficient levelflag related to whether or not the transform coefficient level for thequantized transform coefficient is greater than a second thresholdvalue.
 4. The image decoding method of claim 3, wherein the deriving thequantized transform coefficient comprises: decoding the first transformcoefficient level flag and decoding the parity level flag; and derivingthe quantized transform coefficient based on a value of the decodedparity level flag and a value of the decoded first transform coefficientlevel flag, and wherein the decoding the first transform coefficientlevel flag is performed prior to the decoding the parity level flag. 5.The image decoding method of claim 1, wherein the residual informationis derived through different syntax elements based on whether or not thetransform is applied to the current block.
 6. An image encoding methodperformed by an encoding apparatus, the method comprising: deriving aresidual sample for a current block; deriving a transform coefficientfor the current block based on the residual sample; deriving a quantizedtransform coefficient for the current block by performing a quantizationfor the transform coefficient, and encoding residual informationincluding information on the quantized transform coefficient to output abitstream, wherein the residual information includes transformcoefficient level flags related to whether or not a transformcoefficient level for the quantized transform coefficient is greaterthan predetermined thresholds, and wherein a number of the transformcoefficient level flags is based on whether or not a transform isapplied to the current block.
 7. The image encoding method of claim 6,wherein the residual information includes a first transform coefficientlevel flag related to whether or not the transform coefficient level forthe quantized transform coefficient is greater than a first thresholdvalue, and a second transform coefficient level flag related to whetheror not the transform coefficient level for the quantized transformcoefficient is greater than a second threshold value, and wherein thesecond transform coefficient level flag is decoded in different waysbased on whether or not the transform is applied to the current block.8. The image encoding method of claim 6, wherein the residualinformation includes a context syntax element coded based on a context,and wherein the context syntax element includes a significantcoefficient flag related to whether or not the quantized transformcoefficient is a non-zero significant coefficient, a parity level flagrelated to a parity of the transform coefficient level for the quantizedtransform coefficient, a first transform coefficient level flag relatedto whether or not the transform coefficient level for the quantizedtransform coefficient is greater than a first threshold value, and asecond transform coefficient level flag related to whether or not thetransform coefficient level for the quantized transform coefficient isgreater than a second threshold value.
 9. The image encoding method ofclaim 8, wherein the deriving the quantized transform coefficientcomprises: encoding the first transform coefficient level flag andencoding the parity level flag; and deriving the quantized transformcoefficient based on a value of the parity level flag and a value of thefirst transform coefficient level flag, and wherein the encoding thefirst transform coefficient level flag is performed prior to theencoding the parity level flag.
 10. The image encoding method of claim6, wherein the residual information is derived through different syntaxelements based on whether or not the transform is applied to the currentblock.
 11. A non-transitory computer-readable digital storage mediumstoring a bitstream generated by an image encoding method, the methodcomprising: deriving a residual sample for a current block; deriving atransform coefficient for the current block based on the residualsample; deriving a quantized transform coefficient for the current blockby performing a quantization for the transform coefficient, and encodingresidual information including information on the quantized transformcoefficient to output the bitstream, wherein the residual informationincludes transform coefficient level flags related to whether or not atransform coefficient level for the quantized transform coefficient isgreater than predetermined thresholds, and wherein a number of thetransform coefficient level flags is based on whether or not a transformis applied to the current block.
 12. The non-transitorycomputer-readable digital storage medium of claim 11, wherein theresidual information includes a first transform coefficient level flagrelated to whether or not the transform coefficient level for thequantized transform coefficient is greater than a first threshold value,and a second transform coefficient level flag related to whether or notthe transform coefficient level for the quantized transform coefficientis greater than a second threshold value, and wherein the secondtransform coefficient level flag is decoded in different ways based onwhether or not the transform is applied to the current block.
 13. Thenon-transitory computer-readable digital storage medium of claim 11,wherein the residual information includes a context syntax element codedbased on a context, and wherein the context syntax element includes asignificant coefficient flag related to whether or not the quantizedtransform coefficient is a non-zero significant coefficient, a paritylevel flag related to a parity of the transform coefficient level forthe quantized transform coefficient, a first transform coefficient levelflag related to whether or not the transform coefficient level for thequantized transform coefficient is greater than a first threshold value,and a second transform coefficient level flag related to whether or notthe transform coefficient level for the quantized transform coefficientis greater than a second threshold value.
 14. The non-transitorycomputer-readable digital storage medium of claim 13, wherein thederiving the quantized transform coefficient comprises: encoding thefirst transform coefficient level flag and encoding the parity levelflag; and deriving the quantized transform coefficient based on a valueof the parity level flag and a value of the first transform coefficientlevel flag, and wherein the encoding the first transform coefficientlevel flag is performed prior to the encoding the parity level flag.